Make "ghc-pkg field pkg field --simple-output" do something useful

It used to just ignore the --simple-output flag
Remove some old "backwards compatibility" code

It was marked as "backwards compatibility" in 2006, so I think
can be removed now. It allowed using old field names when looking
at fields with ghc-pkg.
Update Cabal
Small refactoring in ghc-pkg
Tidy up some build system code
Add -L flags to libs.depend (fixes #7465)

Based on a patch from Peter Trommler
Fix build