MSBuild SolutionParser interprets solution folders as project files

### What You Are Seeing?

When parsing a solution, solution folders are added to SolutionParserResult.Files as ProjectFile objects with the FilePath set to the name of the solution folder name.
### What is Expected?

The solution folders would be either ignored, or represented as a different type than ProjectFile.
### What version of Cake are you using?

0.15.2+Branch.main.Sha.c2ca4f7ca51c59dc9c47c9830125b4e66cf14e7a
### How Did You Get This To Happen? (Steps to Reproduce)

Sample cake snippet

``` csharp
var parsedSolution = ParseSolution("SampleSolution.sln");
var parsedProjects = parsedSolution.Projects.Select(p => ParseProject(p.Path)).ToList();
```

Sample solution snippet

```
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "SampleProject", "src\SampleProject\SampleProject.csproj", "{DE89E10E-1793-4D8C-81C8-12982607B2D9}"
EndProject
Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "src", "src", "{0C3E1B23-CF55-49C7-8C1D-38FA8DFC7F06}"
EndProject
```
### Output Log

```
Compiling build script...
Error: Cake.Core.CakeException: Project file 'C:/code/cake-test/src' do not exist.
at Cake.Common.Solution.Project.ProjectParser.Parse(FilePath projectPath) in C:\code\github\cake\src\Cake.Common\Solution\Project\ProjectParser.cs:line 68
at Cake.Common.Solution.Project.ProjectAliases.ParseProject(ICakeContext context, FilePath projectPath) in C:\code\github\cake\src\Cake.Common\Solution\Project\ProjectAliases.cs:line 66
at Submission#0.ParseProject(FilePath projectPath)
at Submission#0.<.ctor>b__0(SolutionProject p)
at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
at Submission#0..ctor(Session session, Object& submissionResult)
at Submission#0.(Session session)
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.Scripting.Roslyn.Stable.DefaultRoslynScriptSession.Execute(Script script) in C:\code\github\cake\src\Cake\Scripting\Roslyn\Stable\DefaultRoslynScriptSession.cs:line 33
at Cake.Core.Scripting.ScriptRunner.Run(IScriptHost host, FilePath scriptPath, IDictionary`2 arguments) in C:\code\github\cake\src\Cake.Core\Scripting\ScriptRunner.cs:line 201
at Cake.Commands.BuildCommand.Execute(CakeOptions options) in C:\code\github\cake\src\Cake\Commands\BuildCommand.cs:line 34
at Cake.CakeApplication.Run(CakeOptions options) in C:\code\github\cake\src\Cake\CakeApplication.cs:line 46
at Cake.Program.Main() in C:\code\github\cake\src\Cake\Program.cs:line 75
```

Start at Cake.Common.Solution.Project.ProjectParser.Parse, referenced at ProjectParser.cs line 68, and trace how SolutionParserResult.Files is populated from the sample .sln entries. Reproduce the ParseSolution/ParseProject sequence, then ensure solution folders are not treated as ProjectFile objects or are represented distinctly.

Written by the indexing model from the issue text.