Unable to use custom static classes with extension methods in cake scripts

### What You Are Seeing?

I have added a static class containing extension methods for the `AssemblyInforParseResult` class in order to simplify the instantiation of an `AssemblyInfoSettings` object. When running the build, I get the following message:

``` sh
Error: M:/Projects/Dev/CakeTest/common.cake(34,40): error CS1109: Extension methods must be defined in a top level static class; VersioningHelper is a nested class
```
### What is Expected?

I would expect the build script to be compiled without errors.
I am guessing this happens because Cake converts the entire cake.build into a single class that is then complied with Roslyn, thus my custom class turns into a nested class, and, correctly, the compiler doesn't allow declaring extension methods in it. I am able to circumvent this by creating a class library with the custom functionality and referencing it, but this is not very convenient as the cake script is no longer self-contained.
### What version of Cake are you using? Are you running on a 32 or 64 bit system? What environment are you running on? Windows? Linux? Mac? Are you running on a CI Server? If so, which one?

I am running Cake 0.16.2 on Windows (64bit), both locally and on (on-premise) TFS.
### How Did You Get This To Happen? (Steps to Reproduce)

Add a static class containing extension methods to a cake script
### Output Log

``` sh
Module directory does not exist.
Analyzing build script...
Analyzing M:/Projects/Dev/CakeTest/build.cake...
Analyzing M:/Projects/Dev/CakeTest/common.cake...
Processing build script...
Installing addins...
Package Cake.FileHelpers has already been installed.
The addin Cake.FileHelpers will reference Cake.FileHelpers.dll.
Creating script session...
Adding reference to mscorlib.dll...
Adding reference to System.dll...
Adding reference to System.Core.dll...
Adding reference to System.Data.dll...
Adding reference to System.Xml.dll...
Adding reference to System.Xml.Linq.dll...
Adding reference to Cake.Core.dll...
Adding reference to Cake.Common.dll...
Adding reference to Cake.exe...
Adding reference to Cake.FileHelpers.dll...
Importing namespace Cake.Common...
Importing namespace Cake.Common.Build...
Importing namespace Cake.Common.Build.AppVeyor...
Importing namespace Cake.Common.Build.AppVeyor.Data...
Importing namespace Cake.Common.Build.BitbucketPipelines...
Importing namespace Cake.Common.Build.BitbucketPipelines.Data...
Importing namespace Cake.Common.Build.Bitrise...
Importing namespace Cake.Common.Build.Bitrise.Data...
Importing namespace Cake.Common.Build.ContinuaCI...
Importing namespace Cake.Common.Build.ContinuaCI.Data...
Importing namespace Cake.Common.Build.Jenkins...
Importing namespace Cake.Common.Build.Jenkins.Data...
Importing namespace Cake.Common.Build.TravisCI...
Importing namespace Cake.Common.Build.TravisCI.Data...
Importing namespace Cake.Common.Diagnostics...
Importing namespace Cake.Common.IO...
Importing namespace Cake.Common.IO.Paths...
Importing namespace Cake.Common.Net...
Importing namespace Cake.Common.Security...
Importing namespace Cake.Common.Solution...
Importing namespace Cake.Common.Solution.Project...
Importing namespace Cake.Common.Solution.Project.Properties...
Importing namespace Cake.Common.Solution.Project.XmlDoc...
Importing namespace Cake.Common.Text...
Importing namespace Cake.Common.Tools...
Importing namespace Cake.Common.Tools.Cake...
Importing namespace Cake.Common.Tools.Chocolatey...
Importing namespace Cake.Common.Tools.Chocolatey.ApiKey...
Importing namespace Cake.Common.Tools.Chocolatey.Config...
Importing namespace Cake.Common.Tools.Chocolatey.Features...
Importing namespace Cake.Common.Tools.Chocolatey.Install...
Importing namespace Cake.Common.Tools.Chocolatey.Pack...
Importing namespace Cake.Common.Tools.Chocolatey.Pin...
Importing namespace Cake.Common.Tools.Chocolatey.Push...
Importing namespace Cake.Common.Tools.Chocolatey.Sources...
Importing namespace Cake.Common.Tools.Chocolatey.Upgrade...
Importing namespace Cake.Common.Tools.DNU...
Importing namespace Cake.Common.Tools.DNU.Build...
Importing namespace Cake.Common.Tools.DNU.Pack...
Importing namespace Cake.Common.Tools.DNU.Restore...
Importing namespace Cake.Common.Tools.DotCover...
Importing namespace Cake.Common.Tools.DotCover.Analyse...
Importing namespace Cake.Common.Tools.DotCover.Cover...
Importing namespace Cake.Common.Tools.DotNetCore...
Importing namespace Cake.Common.Tools.DotNetCore.Build...
Importing namespace Cake.Common.Tools.DotNetCore.Execute...
Importing namespace Cake.Common.Tools.DotNetCore.Pack...
Importing namespace Cake.Common.Tools.DotNetCore.Publish...
Importing namespace Cake.Common.Tools.DotNetCore.Restore...
Importing namespace Cake.Common.Tools.DotNetCore.Run...
Importing namespace Cake.Common.Tools.DotNetCore.Test...
Importing namespace Cake.Common.Tools.DupFinder...
Importing namespace Cake.Common.Tools.Fixie...
Importing namespace Cake.Common.Tools.GitLink...
Importing namespace Cake.Common.Tools.GitReleaseManager...
Importing namespace Cake.Common.Tools.GitReleaseManager.AddAssets...
Importing namespace Cake.Common.Tools.GitReleaseManager.Close...
Importing namespace Cake.Common.Tools.GitReleaseManager.Create...
Importing namespace Cake.Common.Tools.GitReleaseManager.Export...
Importing namespace Cake.Common.Tools.GitReleaseManager.Publish...
Importing namespace Cake.Common.Tools.GitReleaseNotes...
Importing namespace Cake.Common.Tools.GitVersion...
Importing namespace Cake.Common.Tools.ILMerge...
Importing namespace Cake.Common.Tools.ILRepack...
Importing namespace Cake.Common.Tools.InspectCode...
Importing namespace Cake.Common.Tools.MSBuild...
Importing namespace Cake.Common.Tools.MSTest...
Importing namespace Cake.Common.Tools.NSIS...
Importing namespace Cake.Common.Tools.NuGet...
Importing namespace Cake.Common.Tools.NuGet.Install...
Importing namespace Cake.Common.Tools.NuGet.Pack...
Importing namespace Cake.Common.Tools.NuGet.Push...
Importing namespace Cake.Common.Tools.NuGet.Restore...
Importing namespace Cake.Common.Tools.NuGet.SetApiKey...
Importing namespace Cake.Common.Tools.NuGet.SetProxy...
Importing namespace Cake.Common.Tools.NuGet.Sources...
Importing namespace Cake.Common.Tools.NuGet.Update...
Importing namespace Cake.Common.Tools.NUnit...
Importing namespace Cake.Common.Tools.OctopusDeploy...
Importing namespace Cake.Common.Tools.OpenCover...
Importing namespace Cake.Common.Tools.ReportGenerator...
Importing namespace Cake.Common.Tools.ReportUnit...
Importing namespace Cake.Common.Tools.Roundhouse...
Importing namespace Cake.Common.Tools.SignTool...
Importing namespace Cake.Common.Tools.SpecFlow...
Importing namespace Cake.Common.Tools.SpecFlow.StepDefinitionReport...
Importing namespace Cake.Common.Tools.SpecFlow.TestExecutionReport...
Importing namespace Cake.Common.Tools.TextTransform...
Importing namespace Cake.Common.Tools.VSTest...
Importing namespace Cake.Common.Tools.WiX...
Importing namespace Cake.Common.Tools.WiX.Heat...
Importing namespace Cake.Common.Tools.XBuild...
Importing namespace Cake.Common.Tools.XUnit...
Importing namespace Cake.Common.Xml...
Importing namespace Cake.Core...
Importing namespace Cake.Core.Diagnostics...
Importing namespace Cake.Core.IO...
Importing namespace Cake.Core.Scripting...
Importing namespace Cake.FileHelpers...
Importing namespace System...
Importing namespace System.Collections.Generic...
Importing namespace System.IO...
Importing namespace System.Linq...
Importing namespace System.Text...
Importing namespace System.Threading.Tasks...
Compiling build script...
Error: Roslyn.Compilers.CompilationErrorException: M:/Projects/Dev/CakeTest/common.cake(34,40): error CS1109: Extension methods must be defined in a top level static class; VersioningHelper is a nested class
at Roslyn.Scripting.CommonScriptEngine.CompilationError(DiagnosticBag localDiagnostics, DiagnosticBag diagnostics)
at Roslyn.Scripting.CommonScriptEngine.Compile(String code, String path, DiagnosticBag diagnostics, Session session, Type delegateType, Type returnType, CancellationToken cancellationToken, Boolean isInteractive, Boolean isExecute, CommonCompilation& compilation, Delegate& factory)
at Roslyn.Scripting.CommonScriptEngine.Execute[T](String code, String path, DiagnosticBag diagnostics, Session session, Boolean isInteractive)
at Roslyn.Scripting.Session.Execute(String code)
at Cake.Core.Scripting.ScriptRunner.Run(IScriptHost host, FilePath scriptPath, IDictionary`2 arguments)
at Cake.Commands.BuildCommand.Execute(CakeOptions options)
at Cake.CakeApplication.Run(CakeOptions options)
at Cake.Program.Main()
```

Reproduce the failure with Cake 0.16.2 using a self-contained script like common.cake, then inspect Cake.Core.Scripting.ScriptRunner and the Roslyn script-compilation path described in the stack trace. Done means a cake script can declare a top-level static class with extension methods and compile without CS1109.

Written by the indexing model from the issue text.