Bring back live host change

[agocke]

The previous problem seems to be the usage of RuntimeFiles as the ItemGroup for the host files instead of NativeRuntimeAsset, and the lack of PackOnly, which prevents hostfxr from being included in the shared framework package.

[dotnet-issue-labeler]

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

[agocke]

also @jkoritzinsky

[ViktorHofer]

@agocke in general looks good but can you please point me to the fix (in this change) for the previous regression?

[agocke]

@ViktorHofer This is the problematic part. By moving the host files to RuntimeFiles they ended up getting copied into the shared framework pack, when they were only supposed to be in the runtime pack. Keeping this in the Microsoft.NetCore.App package fixed the problem.

[agocke]

Also, the changes in externals.proj basically replicate the behavior that was intended with the above change.

[ViktorHofer]

If it works, awesome 👍

@vitek-karas do you know if the host packages (Microsoft.NETCore.DotNetHost and Microsoft.NETCore.DotNetHostPolicy) are still consumed by anyone? Asking as this PR removes our own dependency on them.

[agocke]

@elinor-fung Mentioned that the host packages aren't directly depended on at the moment, but they're our vehicle for symbols for the hosting components. I'd want to bring the change through to stop building them separately, and confirm we still have symbols.