This is a work in progress and very much incomplete, although good enough for simple uses (like, getting me off of ancient Debian packages). It knows about the packages provided by hvr's PPA.
Currently this must be installed manually by symlinking ghc-wrapper.pl
to the names of various tools as listed in %pkgs.
You use it by setting $USEGHC (or $USEHAPPY, $USEALEX, $USECABAL) to
an appropriate version found under /opt/{ghc,happy,alex,cabal}. By default
it will use the latest release version it finds there. You can also set it to
"-" to force use of a version found on $PATH; it will detect and ignore
itself to avoid infinite loops.