Clearing up confusion about package keys, package ids, et al

I have written a new wiki page which tries to explicate these concepts: https://ghc.haskell.org/trac/ghc/wiki/Commentary/Packages/Concepts I haven't finished writing my proposal but one thing that I am going to argue is that we are going to need *MORE* package identification concepts (sorry Simon!); e.g. reusing package keys as Nix keys just isn't going to work. Edward