In order to simplify the task, the version munging logic has
been radically simplified:

Previously, in cases where the version contained dates as version components,
the build-system would munge the version of the stage1 ghc package before
registering the `ghc` package.

However, this hack was already questionable at the time of its introduction
(c.f. 7b45c46c).
Simplifying the build-systems by avoiding such hacks may also help the
shaking-up-ghc effort.

So now we simply munge directly via the `.cabal` files, which gives a simpler
picture, as now every stage is munged the same. Munging is only active when
the first patch-level version component is a date. So stable snapshots and release
candidates are unaffacted (as those have the date in the second patch-level
version component)

Reviewers: simonmar, bgamari, austin, thomie, ezyang

Reviewed By: bgamari, thomie, ezyang

Differential Revision: https://phabricator.haskell.org/D1673