Refactor CrossGen build to better support arbitrary RIDs

[MichaelSimons]

This issue stems from review feedback from this PR.

The general issue is that the CrossGen project has a hard coded RID list it supports. This hard coded list doesn't work for source-build, which needs to build arbitrary non-portable RIDs like centos.8-x64 and banana.42-x64 with no unexpected feature gaps vs. portable RIDs. Currently, source-build conditionally disables the hard coded RID list but this could be improved. It could have a list of "good" portable RIDs, and check if the current RID is a descendant of any of the "good" RIDs in the RID graph. Then, there would be no need for source-build-specific behavior, because banana.42-x64 derives from linux-x64.

[ViktorHofer]

cc @trylek @ericstj @dagood @jkoritzinsky