Add test project which finds and validates CMake packages and config.

[stellaraccident]

Includes changes needed to get the packaging working and passing a test.

• Add a base/aux-overlay that populates some structural files that don't belong to any project:
  • CMake package redirection in the root lib/cmake directory to packages in the lib/llvm/lib/cmake directory (so that users only need to specify one CMAKE_PREFIX_PATH).
  • An llvm -> lib/llvm symlink to account for an old directory organization that is still required in some cases (should be removed soon in favor of fixing upstream).
• Add default_patterns boolean to artifact component descriptors, disabling the use of default includes/excludes for a component.
• Build therock-dist(-.+)? as part of ALL.
• Adds amd-comgr and hipcc to the amd-llvm artifact bundle (was an oversight before).
• Includes hipBLAS-common in the blas artifact bundle (was an oversight before).

Fixes #38