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

[Edward Z. Yang]

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