externals.csproj is the remaining depproj file that we want to get rid of and replace it with using live assets where applicable. The project is currently responsible for:

• Binplacing the Microsoft.DiaSymReader.Native package into the shared framework.
  - Restoring the host packages and binplace them into the testhost and runtime folder.
• Copy runtime (coreclr & mono) assets into the testhost and runtime folder.
• Swap IL version of corelib with native one.

Most/all of this work could be replaced by leveraging the same infrastructure that the installer tests use to assemble the test layout to test against. Also see discussion in https://github.com/dotnet/runtime/pull/40172/files#r465090284.

cc @ericstj @jkoritzinsky @akoeplinger @safern @Anipik