It should work to write an indefinite package using TemplateHaskell,
so long as all of the actual TH code lives outside of the package.
However, cleverness we had to build TH code even when building
with -fno-code meant that we attempted to build object code for
modules in an indefinite package, even when the signatures were
not instantiated.  This patch disables said logic in the event
that an indefinite package is being typechecked.

Signed-off-by: Edward Z. Yang <ezyang@fb.com>

Test Plan: validate

Reviewers: simonpj, bgamari

Reviewed By: bgamari

Subscribers: rwbarton, carter

GHC Trac Issues: #16219

Differential Revision: https://phabricator.haskell.org/D5475

(cherry picked from commit d6d735c1)