Stub out pkgState with non-error, helps with debugging.

Summary:
When we use -dppr-debug, we'll sometimes attempt to
hit the pkgState to give more detailed information.  It's pretty
annoying to have this fail because we haven't filled in the
pkgState: we can still get useful debugging information by
printing the raw package key.  To avoid this, we just put
in some empty stub pkgState, to be filled in later.

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

Test Plan: validate

Reviewers: simonpj, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D817