There used to be a lot of custom make code to build ghc-pkg with the
stage0 compiler. Commit ac5a3145
thankfully cleaned this up, by using the build settings from the
ghc-pkg.cabal file.

This commit removes some remains of the old way of installing ghc-pkg
when Stage1Only=YES. Notably, we called both `build-prog` as
`shell-wrapper`. This is surely wrong, because `build-prog` already
calls `shell-wrapper`. It isn't needed to set WANT_INSTALLED_WRAPPER
either; build-prog does that for us.

This prevents the following warnings when Stage1Only=YES:
  utils/ghc-pkg/ghc.mk:46: warning:
    overriding commands for target `install_utils/ghc-pkg_dist_wrapper'
  utils/ghc-pkg/ghc.mk:37: warning:
    ignoring old commands for target `install_utils/ghc-pkg_dist_wrapper'

Also add more comments and restructure a bit.

Reviewed by: austin

Differential Revision: https://phabricator.haskell.org/D1063