Previously the post-build phase was just reportBuildFailures and was
called directly from Cmd{Build,Repl}. So initially this just rearranges
things without changing behaviour, but it gives us a place to add other
post-build actions.

This also simplifies the code in Cmd{Configure,Build,Repl}.

A few examples are mentioned in a comment, including updating:
  .ghc.environment
  bin symlinks/wrappers
  haddock/hoogle/ctags indexes
  delete stale lib registrations
  delete stale package dirs