Account for special GHC.Prim import in warnUnusedPackages

The GHC.Prim import is treated quite specially primarily because there isn't an interface file for GHC.Prim. Therefore we record separately in the ModSummary if it's imported or not so we don't go looking for it.

This logic hasn't made it's way to -Wunused-packages so if you imported GHC.Prim then the warning would complain you didn't use -package ghc-prim.

Fixes #23212 (closed)