Fix #11256 by not immediately erroring if we can't find a module.

Test Plan: validate

Reviewers: austin, bgamari, thomie

Reviewed By: bgamari, thomie

Differential Revision: https://phabricator.haskell.org/D1669

GHC Trac Issues: #11256

compiler/main/GhcMake.hs

@@ -1886,13 +1886,11 @@ summariseModule hsc_env old_summary_map is_boot (L loc wanted_mod)
                 | isJust (ml_hs_file location) ->
                         -- Home package
                          just_found location mod
-                | otherwise ->
-                        -- Drop external-pkg
-                        ASSERT(moduleUnitId mod /= thisPackage dflags)
-                        return Nothing
 
-             err -> return $ Just $ Left $ noModError dflags loc wanted_mod err
+             _ -> return Nothing
                         -- Not found
+                        -- (If it is TRULY not found at all, we'll
+                        -- error when we actually try to compile)
 
     just_found location mod = do
                 -- Adjust location to point to the hs-boot source file,

testsuite/tests/cabal/cabal07/cabal07.stderr

 
-Q.hs:3:8: error:
-    Could not find module ‘Data.Set’
-    It is a member of the hidden package ‘containers-0.5.6.3@containers-0.5.6.3’.
+Q.hs:3:1: error:
+    Failed to load interface for ‘Data.Set’
+    It is a member of the hidden package ‘containers-0.5.7.0’.
     Perhaps you need to add ‘containers’ to the build-depends in your .cabal file.
     Use -v to see a list of the files searched for.

testsuite/tests/driver/driver063.stderr

 
-D063.hs:2:8:
-    Could not find module ‘A063’
+D063.hs:2:1: error:
+    Failed to load interface for ‘A063’
     It is not a module in the current program, or in any known package.

testsuite/tests/ghc-e/should_run/T2636.stderr

 
-T2636.hs:1:8:
-    Could not find module ‘MissingModule’
+T2636.hs:1:1: error:
+    Failed to load interface for ‘MissingModule’
     Use -v to see a list of the files searched for.