Support building sdks that can build self-contained / ReadyToRun / NativeAOT applications without using pre-builts from nuget.org

[tmds]

Thanks to the reference packs, a source-build sdk can now build .NET Core applications that can be considered themselves source-buildable.

This doesn't apply to self-contained applications, and ReadyToRun applications. These require downloading a Microsoft.NETCore.App.Runtime.<rid> package from nuget.org. (This includes framework dependent ReadyToRun, probably because it uses the package to aquire crossgen.)

This issue is for building and including the Runtime package with the sdk. This makes it possible for the maintainer to apply patches during source-build, and the applications built this way using the sdk can be considered source-buildable.

cc @dagood @nguerrera @omajid

[dagood]

This issue is for building and including the Runtime package with the sdk.

Runtime packs are similar to targeting packs, so IMO they should be included in e.g. dotnet-runtime-pack-x-y packages, which the SDK package would have a dependency on.

This makes it possible for the maintainer to apply patches during source-build, and the applications built this way using the sdk can be considered source-buildable.

A note about this from #1202, to put it in context: the runtime pack has limitations on what it supports, due to glibc compatibility and portable vs. non-portable builds. An SCD publish for linux-x64 would need to bring down the extra-compatible prebuilt, but e.g. fedora.30-x64 would be source-built.

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I believe the SDK and perhaps Core-SDK repos will need to add support for this. The SDK currently locates targeting packs with TargetingPackRoot ($(NetCoreTargetingPackRoot)) which points to e.g. /usr/bin/dotnet/packs/. The runtime pack resolver task doesn't seem to have anything similar.

The definition of NetCoreTargetingPackRoot is over in Core-SDK.

The apphost pack resolver uses TargetingPackRoot too, despite the name, so I guess it might be fine to use it for runtime packs too.

[tmds]

It makes sense to make it work similar to the apphost package.

Can we set a target milestone for this?

I can probably implement this based on these references.

[nguerrera]

Adding @dsplaisted