Put package assets in TFM-specific folder

[wtgodbe]

Fixes #58990
Related: NuGet/Home#6932

When a package has a build or buildmultitargeting folder without a TFM subfolder, NuGet considers it to be compatible with any TFM. This is causing builds that should be failing with restore errors, to instead later hit a less helpful runtime error.

[wtgodbe]

Test build to get packages: https://dev.azure.com/dnceng/internal/_build/results?buildId=2582989&view=results

[adityamandaleeka]

Thanks for reporting this @loganmj and thanks @BrennanConroy and @wtgodbe for the quick investigation/fix!

[captainsafia]

Does ApiDescription.Server need a similar treatment?

[wtgodbe]

Does ApiDescription.Server need a similar treatment?

It does, nice catch. Plus I need to fix up Microsoft.AspNetCore.Identity.UI. Will do that today

[wtgodbe]

Actually, not sure if we can fix APIDescription.Server since it targets netcoreapp2.1, $(DefaultNetCoreTargetFramework), and $(DefaultNetFxTargetFramework), and the files in build/buildMultiTargeting are meant to be used on any of those. @ericstj how has dotnet/runtime solved similar problems (packages with non-TFM'd build folders are considered compatible with any TFM by nuget)

[ericstj]

We make sure to use TFMs with all assets, including build, buildTransitive.

I don't think an of our packages use buildMultiTargeting -- I can see why you can't use a framework folder there since it's part of the outer build. I think in that case you'd want to make sure your targets raised the appropriate Diagnostics when used in an unsupported project type.