Sanity check if we pick up an hsig file without -instantiated-with.
Summary: Previously we would just let compilation proceed along until we tried to pull up the Module for the hsig file, and get main:A instead of <A>, and get a mysterious error. Check for this earlier! Fixes #12955. Signed-off-by: Edward Z. Yang <email@example.com> Test Plan: validate Reviewers: simonpj, austin, bgamari Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D2815 GHC Trac Issues: #12955
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