Add build assets to the TFM compatibility check and assets file

[emgarten]

The guardrails compatibility check for target frameworks is built for xproj/project.json and ignores build assets.

With the move to MSBuild NuGet should include build assets during the compat check, and it should also write these items to the lock file to make it clear that they are assets selected from the package.

[karann-msft]

Issue restoring MSBuild only NuGet package.pdf

[emgarten]

Fixed with NuGet/NuGet.Client@efae789