Simplify ghc-boot database representation with new type class.

Previously, we had an 'OriginalModule' type in ghc-boot which
was basically identical to 'Module', and we had to do a bit of
gyrating to get it converted into the right form.  This commit
introduces a new typeclass, 'DbModuleRep' which represents types
which we know how to serialize to and from the (now renamed) 'DbModule'
type.

The upshot is that we can just store 'Module's DIRECTLY in
the 'InstalledPackageInfo', no conversion needed.

I took the opportunity to clean up ghc-pkg to make its use of
the 'BinaryStringRep' classes more type safe.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1811

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