Cast deps to formula before passing them to superenv

Dependency names retain the "tap prefix", e.g. the "homebrew/dupes"
part of "homebrew/dupes/zlib". However formula objects do not, and this
is desired because we do not record the tap name as part of the
installation prefix.

So we need to ensure the correct dep names are passed to superenv,
otherwise it will not add the correct directories to various environment
variables.