Design-time builds in CPS are doing too much work because BuildingProject is set to true

To reduce build times, CPS groups build targets for a project together. This has the unintended side-effect of turning `$(BuildingProject)` on during lots of CPS design-time builds.

Lots of behavior in common targets is driven by `$(BuildingProject)`, it's basically the legacy equivalent of `$(DesignTimeBuild)` != 'true'":

```
186>------ Build started: Project: Microsoft.VisualStudio.ProjectSystem.Managed, Targets: CollectResolvedSDKReferencesDesignTime, DebugSymbolsProjectOutputGroup, CollectPackageReferences, ResolveComReferencesDesignTime, ContentFilesProjectOutputGroup, DocumentationProjectOutputGroupDependencies, SGenFilesOutputGroup, ResolveProjectReferencesDesignTime, SourceFilesProjectOutputGroup, DebugSymbolsProjectOutputGroupDependencies, SatelliteDllsProjectOutputGroup, BuiltProjectOutputGroup, SGenFilesOutputGroupDependencies, ResolveAssemblyReferencesDesignTime, CollectAnalyzersDesignTime, CollectSDKReferencesDesignTime, DocumentationProjectOutputGroup, PriFilesOutputGroup, BuiltProjectOutputGroupDependencies, ResolvePackageDependenciesDesignTime, SatelliteDllsProjectOutputGroupDependencies, SDKRedistOutputGroup, CompileDesignTime, CollectResolvedCompilationReferencesDesignTime ------

...

186>Target "BuildOnlySettings" in file "C:\Program Files (x86)\Microsoft Visual Studio\Preview\Enterprise\MSBuild\15.0\Bin\Microsoft.Common.CurrentVersion.targets":
186> Set Property: BuildingProject=true
```

`BuildOnlySettings` is basically a dependency of most of the output groups targets.

Things that I'm seeing we're doing:

- ResolveAssemblyReferences; FindDependencies, FindSatellites, FindSerializationAssemblies, FindRelatedFiles is all true.
- ResolveAssemblyReferences; Silent is false - undecided if this is good or bad.

This would explain why ResolveAssemblyReferences is showing up on traces, FindDependencies is very slow.