PackageValidation task should not filter package assets.

[joperezr]

cc: @Anipik @safern @ericstj

While testing package validation I noticed that apparently we are filtering the assets that we consider when parsing a package here:

sdk/src/Compatibility/Microsoft.DotNet.PackageValidation/NupkgParser.cs

Line 45 in 2347c72

return new Package(packageId, version, packageReader.GetFiles()?.Where(t => t.EndsWith(packageId + ".dll")), packageDependencies, runtimeGraph);

The problem with this, is that we will only consider those filtered files when calculating the best compile and runtime asset to verify APICompat for, but that is not always accurate, as a package could have an asset like lib/net461/_._ which should win as a runtime asset over a lib/netstandard2.0/Foo.dll when evaluating for net461, and that is not happening today. This is important as the previously mentioned example, means that effectively I would be intentionally dropping support for net461, but package validation won't catch it as it would evaluate APICompat against the netstandard2.0 asset.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[joperezr]

We should remove that filtering, and then refactor the code so that if the ref or runtime asset picked for a specific framework is not a .dll (like in the case of a _._ getting picked) then we should not try to read into the assembly and instead just provide the appropriate validation warning saying that the support was dropped. It would also be good to test against packages that produce .winmds instead of .dlls, as I'm not so sure that our current logic would work with them.

[ViktorHofer]

dotnet/aspnetcore#744 (comment) has more details around what a placeholder file means.