Simplify package dump for -v4

Summary:
Previously, on -v4  and greater, we dumped out the entire package
database, including lots of metadata that GHC doesn't really care about,
and is guaranteed to correspond to the equivalent in the local/global
package databases on disk.  So, to make this output more useful, on -v4
we instead just print package IDs, and the exposed and trusted flags
(E and T, which can be tweaked at runtime).

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

Test Plan: successful validate

Reviewers: simonpj

Subscribers: simonmar, relrod

Differential Revision: https://phabricator.haskell.org/D24