Runtime pack support for .NET Core 3.0

[dsplaisted]

For .NET Core 3.0, we expect to separate targeting packs, which contain the assets needed at build time when targeting .NET Core 3.0 from "runtime packs", which will include the platform-specific runtime components for .NET Core 3 which are deployed with self-contained apps.

This issue tracks the work required in order to support runtime packs. This includes:

• Figuring out the layout of runtime packs
• Resolve runtime pack to use for self-contained publish (including RID)
• Consuming the assets from a runtime pack for self-contained publish
• Creating the runtime packs for each supported shared framework:
  • .NET Core
  • ASP.NET Core
  • Microsoft.DesktopUI
• Potentially, supporting explicit runtime pack acquisition (via a command such as dotnet runtime-pack install Microsoft.NETCore.App -version 2.0.6 -rid linux-x64

[dasMulli]

While acquisition is important, I believe that deleting / cleaning installed runtime packs is also important:

• Clean outdated versions (e.g. 2.0.5 and 2.0.6 are installed: 2.0.5 is no longer needed)
  • Could be done automatically on acquisition of 2.0.6 for the same RID
• Clean by RID (inclusive or exclusive)
• If an SDK installer adds a pack (e.g. win-x64 installer installs win-x64 runtime pack), uninstalling the SDK version should remove the runtime pack as well (ref-counted)

Another important aspect is:

• Where are the runtime packs installed? User or machine wide directory?
  • Can these be consumed via dotnet publish --runtime-packs ~/netcore-runtime-packs?
• Which permissions are necessary to install a runtime pack? If some are installed via the installer, which permissions are necessary to clean them? (See permission issues on macOS with the current NuGetFallbackFolder)