Killing or resurrecting version tags for good

[Edward Z. Yang]

Hello all,

A while back, I was nattering around the parsing code for Cabal,
and I noticed that Cabal still had code for parsing version tags,
but was explicitly *not* printing out version tags.  I proposed
that we should get rid of the parsing code as well, but when
I mentioned this on #ghc Mark Lentczner (cc'ed) mentioned that
they were using version tags to identify different versions during
a build, e.g. what Git version things correspond to.

I am not going to claim to know the right answer to this question,
but in either case, it is terrible that we are PARSING tags but
not PRINTING them.  This leads to terrible error messages, among
other things, such as in this case:

    $ ghc-pkg describe json-0.7-foo
    ghc-pkg: cannot find package json-0.7

(NB: json-0.7 is installed.)

So if we're keeping tags around, at least for the immediate future,
then we should print them; otherwise, we should get rid of them
entirely!

Edward