AssetTargetFallback doesn't work if a package author forgot a TFM on one asset group

[ericstj]

I ran into a few packages which were designed to work only with net40 but the package author didn't constrain their build assets with the same TFM (eg: lib\net40\foo.dll, build\foo.targets). I've also seen similar packages where folks forgot to constrain their dependencies (eg: lib\net40\foo.dll, root dependencies with no TFM).

The result was that package would only install the unconstrained assets and wouldn't trigger AssetTargetFallback. I know this is covered in the last row of https://github.com/NuGet/Home/wiki/Enable-.NET-Core-2.0-projects-to-work-with-.NET-Framework-4.6.1-compatible-packages but I want to raise this as a potential issue.

In the case I hit this issue I was able to use PackageTargetFallback instead but that's not a real solution since it's a global hammer and also something that @rrelyea mentioned NuGet wants to deprecate.

I had some other ideas and thought to propose them here as an alternative:

1. Make AssetTargetFallback always ignore TFM-less matches.
2. Allow a project-wide property that says AssetTargetFallback should ignore TFM-less matches.
3. Allow for per-package-reference metadata that says AssetTargetFallback should ignore TFM-less matches.
4. Allow for per-package-reference metadata to tell NuGet specifically which TFM to evaluate for assets within a package. For example UseAssets="net46" or UseAssets="Dependencies=net46;Compile=net46" etc.

[wtgodbe]

We just hit this in aspnetcore too: dotnet/aspnetcore#58995