FIX #903: mkWWcpr: not a product
This fixes the long-standing bug that prevents some code with mutally-recursive modules from being compiled with --make and -O, including GHC itself. See the comments for details. There are some additional cleanups that were forced/enabled by this patch: I removed importedSrcLoc/importedSrcSpan: it wasn't adding any useful information, since a Name already contains its defining Module. In fact when re-typechecking an interface file we were wrongly replacing the interesting SrcSpans in the Names with boring importedSrcSpans, which meant that location information could degrade after reloading modules. Also, recreating all these Names was a waste of space/time.
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