Remove the dependency on buildtools.

- Makes a number of common test projects SDK-style, and replaces the buildtools prerelease restore target with the restore logic built in to the SDK. For some projects, we currently restore 5.0 assets that are used for netcoreapp3.0 - this required a workaround to switch the TFM in a few cases.
- Replaces the corefx testhost deps file creation logic with that from the SDK's publish logic.
- Uses a new IL sdk (as the old one used to make many redundant file copies), and uses an .ilproj to restore ilasm/ildasm up-front.
- Uses the UpdateVersions logic from arcade instead of buildtools.
This opens up the possibility of further simplifying some of our msbuild imports in a future change.

Commit migrated from https://github.com/dotnet/coreclr/commit/0dbe04c4938d16ceee92cd50bc9a4eb375b7b8aa