Use GenAPI to generate the targeting packs

[MichaelSimons]

Currently the targeting packs are based on IL and utilize IlAsm/IlDasm for building/generating the source. In theory we could utilize GenAPI to generate the targeting packs into c# code.

There are unknowns if GenAPI is mature enough to handle shared framework assemblies. A POC would be needed to prove this out.

APICompat should help validate this change.

The benefits of this would be:

1. Removing the IlAsm/IlDasm dependency. This adds additional bootstrap packages to the source build prep bootstrap process. e.g. this would help simplify the platform onboard process. This also unifies the build process between ref/targeting packs. Currently the targeting pack build infra is tied to Linux (see Support creating new reference packages from a Windows environment #1322)
2. Consolidate the generate tooling to be based on one technology (e.g. GenAPI). Currently the targeting pack generator is not even in the public repo (see Move targeting package ILDasm tooling to public SBRP repo source-build#3091)

[ViktorHofer]

cc @ericstj @carlossanlop

[ericstj]

Definitely support this. I suspect we might have a few issues, but I think both @carlossanlop and @marklio have been doing some work that does run GenAPI through most framework API and rooting those issues out to fix them. I bet we're pretty close to being able to do this.

[marklio]

I did a proof of concept to use GenAPI to replace the old asmmeta system in .NET Framework builds. Once we were satisfied that the proof of concept was feasible, I set it aside to focus on some other priorities. I will probably jump back on that in the next couple of months, but would gladly partner on investments in this space. Our main hurdles were integration into NetFX-based MSBuild environmments and filtering types/members to deal with the cycles in NetFX assembly reference graphs.