Support dotnet pack on libraries projects

[ViktorHofer]

dotnet pack currently does nothing when invoked on an library project. We should be able to route dotnet pack invoked on the source project to the pkgproj and invoke its build target.

In addition to that it would be nice if the pkgproj build output doesn't point to the nuspec but to the nupkg instead (same what NuGet does). //update: already done by @ericstj: dotnet/arcade@b0b76b6

Long term, we should consider getting rid of pkgprojs and enable dotnet pack natively on our source projects. This probably involves extending the existing NuGet infrastructure, presumably with a new msbuild sdk similar to the existing Microsoft.DotNet.Build.Tasks.Packaging infra package. Related: dotnet/arcade#1033

See #37961 for exposing a single entry point for generating the library package.

cc @joperezr @ericstj @Anipik

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[ViktorHofer]

This will help make packaging easier, i.e. #28244