Summary:
New flag to ghc-pkg register/update to lift the restriction on multiple instances of the same package version being in a db at once.

Lifting the restriction is easy. The tricky bit is checking ghc does something sensible, but from the reading of the code it should treat such instances the same way it does with multiple instances between multiple DBs.

We'll also need a way to unregister by installed package id.

Test Plan: need to test that ghc is doing what we expect, at least if you use it like -hide-all-packages -package-id this -package-id that

Reviewers: ezyang, simonmar

Reviewed By: simonmar

Subscribers: relrod

Projects: #ghc

Differential Revision: https://phabricator.haskell.org/D32