ghc-boot-9.0.0.20201227: Shared functionality between GHC and its boot libraries

This library is shared between GHC, ghc-pkg, and other boot libraries.

A note about GHC.Unit.Database: it only deals with the subset of the package database that the compiler cares about: modules paths etc and not package metadata like description, authors etc. It is thus not a library interface to ghc-pkg and is *not* suitable for modifying GHC package databases.

The package database format and this library are constructed in such a way that while ghc-pkg depends on Cabal, the GHC library and program do not have to depend on Cabal.