5.0: .hi files should indicate origin

It would be nice if a .hi file indicated its origin. 
Otherwise, if you happen to have two different files
with the same module name, GHC can get confused (or
more importantly, the user can get confused!), because
it has no way of knowing whether it's using the .hi
file from the wrong source file.  This seems a bit of a
marginal case, but it shows up in things like test
suites.  In GHC's tests, tc056.hs and tc057.hs if run
one after the other (as is intended), will fail on
tc057.hs with a warning about something that comes from
the .hi file of test056.

% ghc -c tc056.hs 

tc056.hs:14:
    Warning: Duplicate class assertion `Eq' a' in the
context:
		 (Eq' a, Eq' a) => ...
% ghc -c tc057.hs 

./ShouldSucceed.hi:8:
    Warning: Duplicate class assertion
`ShouldSucceed.Eq' a' in the context:
		 (ShouldSucceed.Eq' a, ShouldSucceed.Eq' a) => ...

As you might guess, there's no duplicate class
assertion in tc057.hs.

Trac metadata

Trac field	Value
Version	None
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	ResolvedWon'tFix
Component	Compiler
Test case
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