[project @ 1999-01-19 11:39:44 by sof]

mkExportAvails: remove NotAvailable entries corresponding
to 'hidden' entities from 'unqual_avails'. Not needed and
leaving them in causes bother later on in the renamer.

This fixes the problem of compiling

   module B ( module A ) where { import A hiding ( a ) }

Regression test added to check for this - will commit soon.