Use finalized targeting pack format

[dsplaisted]

With #2774, we are resolving assets from "targeting packs", but we are using the existing packages (such as Microsoft.NETCore.App) as the targeting packs. We plan to use different packages with different layouts as the real targeting packs. Along with that, we will need to update the asset resolution logic to account for this.

See dotnet/designs#50 for the current design for the targeting pack layout.

Items that should be resolved from targeting packs:

• Assembly references (from data/FrameworkList.xml)
• Platform manifests for conflict resolution (from data/PlatformManifest.txt)
• Remove code for PackageConflictPreferredPackages (use PackageConflictOverrides instead)
  • See this comment: Get package overrides from txt file in targeting pack instead of msbuild item #2937 (comment)
• PackageConflictOverrides (from data/PackageOverrides.txt)

Status of having these items in targeting packs:

• Microsoft.NETCore.App.Ref
  • FrameworkList.xml
  • PlatformManifest.txt
  • PackageOverrides.txt - https://github.com/dotnet/core-setup/issues/6507
• Microsoft.WindowsDesktop.App.Ref
  • FrameworkList.xml
  • PlatformManifest.txt
  • PackageOverrides.txt - https://github.com/dotnet/core-setup/issues/6507
• NETStandard.Library.Ref (v2.1)
  • FrameworkList.xml
  • PlatformManifest.txt (doesn't apply to .NET Standard)
  • PackageOverrides.txt - https://github.com/dotnet/core-setup/issues/6507
• Microsoft.AspNetCore.App.Ref
  • FrameworkList.xml - Add manifest to ASP.NET targeting pack aspnetcore#10215
  • PlatformManifest.txt
  • PackageOverrides.txt

[nguerrera]

• Platform manifest is already found by convention.
• Get package overrides from txt file in targeting pack instead of msbuild item #2937 tracks the packageconflict items.

[livarcocc]

@dsplaisted to check and close if done. Otherwise specify left over work.