Significant clean-up of the handling of hi-boot files. 
Previously, when compling A.hs, we loaded A.hi-boot, and
it went into the External Package Table.  It was strange
but it worked.  This tidy up stops it going anywhere;
it's just read in, and typechecked into a ModDetails.

All this was on the way to improving the handling of
instances in hs-boot files, something Chris Ryder wanted.
I think they work quite sensibly now.  

If I've got all this right (have not had a chance to
fully test it) we can merge it into STABLE.