Fix recompilation avoidance bug for implementor of hsig.
Summary: I observed a bug where if I modified the module which implemented an hsig in another package, GHC would not recompile the signature in this situation. The root cause was that we were conflating modules from user imports, and "system" module dependencies (from signature merging and instantiation.) So this patch handles them separately. Signed-off-by: Edward Z. Yang <email@example.com> Test Plan: validate Reviewers: simonpj, bgamari, austin Subscribers: rwbarton, thomie, snowleopard Differential Revision: https://phabricator.haskell.org/D3381
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