Hello!

After the most recent issue with Haskell package versions, I was
inspired to finally make it a bit better.  This patch adds a lint
checker to make sure that a Haskell package’s version is not ahead of
the desired Stackage LTS version!  The check itself is nice and simple,
but I also added a test that is anything but.

To make the test work, I made sure our Hackage and Stackage importers
use base URLs that are parameters.  Then, I used ‘with-http-server’ and
‘parameterize’d the URLs to use the local server.  (This all follows the
example of the SWH checker.)  It might be clearer if the test were split
in two, but the Stackage importer memoizes the list of packages sent
from the Stackage server.  That means that the two tests would have to
be run in a certain order to work, which is pretty dodgy.

Is the test too complicated to be worth it?  I might be able to make it
a little clearer, but I’m not sure it’s worth including at all.
Thoughts?


-- Tim