Include path to ghc-pkg in `ghc --info`

haskell/ghcup-hs#73 notices that build tools currently have no reliable way of identifying the hc-pkg executable associated with a given compiler. This is especially problematic when compilers are installed with version suffixes. Cabal currently uses a variety of heuristics to identify ghc-pkg, but this is not hard to break (as pointed out in haskell/ghcup-hs#73).

I suggest that we include an hspkg-path entry in the ghc --info output for use by build tools.