Introduce empty ghc-internals and ghc-experimental packages

Here we introduce a pair of new core packages, ghc-internals (which will eventually contain the internal facets of base's implementation) and ghc-experimental (which will contain experimental library features before they are stabilized and moved into base).

See HF Tech Proposal #51 for details.

TODO

• Add packages to release notes
• Improve description of ghc-experimental package
• Add https://edit.smart-cactus.org/K6Ru7g8mRH6_B3KnXKwYMA# as a Note