On the GHC 8.0 RCs, multiple users reported a very strange error
whereby GHC would complain that the symbols names recorded in interface
files did not match the expected name.  The reason for this is
that they were using an old version of Cabal which chose symbol
names differently from the installed package ID ('id' field) which
the package was to be installed with; GHC 8.0 now mandates that
these coincides.

This change adds a test to ghc-pkg to make sure that 'id' and 'key'
(which is how Cabal previously reported what the symbol name
was supposed to be) match; if they don't match or key is missing,
we assume that the Cabal was too old.

Bikeshed points:

    - Should we offer more information about how to upgrade
      Cabal correctly (i.e. specify a version?)

    - Should we allow for a missing 'key'?  If we allow for
      'key' to be missing, we lose the ability to detect
      Cabal from GHC 7.8 or earlier being used.  If we
      requi...