+-- Update the provenances of everything that is in scope.
+-- We must be careful not to disturb the Module package info
+-- already in the cache. Why not? Consider
+-- module A module M( f )
+-- import M( f ) import N( f)
+-- import N
+-- So f is defined in N, and M re-exports it.
+-- When processing module A:
+-- 1. We read M.hi first, and make a vanilla name N.f
+-- (without reading N.hi). The package info says <THIS>
+-- for lack of anything better.
+-- 2. Now we read N, which update the cache to record
+-- the correct package for N.f.
+-- 3. Finally we update provenances (once we've read all imports).
+-- Step 3 must not destroy package info recorded in Step 2.
+