Remove optimized package lookup, simplifying code.

Summary:
A while back when I was refactoring the package code, I tried to solve
a performance problem by introducing a fastpath for module lookups.  Well,
it turned out the performance problem was unrelated, but I kept the optimization
because it seemed vaguely useful.

In this commit, I remove the optimization because I don't really think it's
buying us much and it increased code complexity.

ToDo: Inline mkModuleToPkgConfGeneric into mkModuleToPkgConfAll

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

Test Plan: validate

Reviewers: simonpj, austin

Reviewed By: austin

Subscribers: thomie, carter

Differential Revision: https://phabricator.haskell.org/D434