Fix invalid reference when using DevPacks inside DevPacks with different versions

[MichielOda]

This pull request refines how solution library NuGet packages are handled in the PackageReferenceProcessor. The main improvement is that solution library packages are now excluded from implicit DLL import directory references, which aligns with their special handling. Corresponding unit tests have been updated to reflect this new logic.

Changes to solution library package handling:

• Updated logic in PackageReferenceProcessor.cs to exclude solution library NuGet packages from implicit DLL import directory references by checking with NuGetHelper.IsSolutionLibraryNuGetPackage.

Test updates:

• Removed solution library package paths from ImplicitDllImportDirectoryReferences in ProcessAsyncTest_SolutionLibraries and ProcessAsyncTest_SolutionLibraries2 unit tests.
• Cleaned up related test data and expected results for solution library packages in ProcessAsyncTest_SolutionLibraries2.
• Minor formatting and comment adjustments for clarity in test cases.