Incompatible handling of NuGet package "build" folder

Newer versions of rules_dotnet brought support for the "build" folder that some NuGet packages have. According to #369, this implementation was based on [NETStandard.Library](https://www.nuget.org/packages/NETStandard.Library) as an example of the expected directory structure (where dlls are under framework and lib/ref directories).

However, not all NuGet packages structure their "build" folder in the same way as NETStandard.Library, e.g. [coverlet.collector](https://www.nuget.org/packages/coverlet.collector) and [NUnit3TestAdapter](https://www.nuget.org/packages/NUnit3TestAdapter) have dlls directly under a framework directory and [Microsoft.TestPlatform.TestHost](https://www.nuget.org/packages/Microsoft.TestPlatform.TestHost) has dlls under framework and architecture directories. rules_dotnet won't collect these dlls and therefore they won't be available to the build.

No source file, test, or entry point is named. Start by locating the build-folder DLL collection logic and compare its behavior with the layouts of NETStandard.Library, coverlet.collector, NUnit3TestAdapter, and Microsoft.TestPlatform.TestHost. Done means DLLs directly under framework directories and under framework/architecture directories are available to the Bazel build.

Written by the indexing model from the issue text.