Fixes #3323 and #3324, ensuring we monitor the project Cabal files.
Original fix by Edward Z. Yang. The approach in this patch is to fix an
underlying problem. Subsequent patches use a more consistent approach to
the monitoring as suggested by Edward.

The motivating example is:

  matches <- matchFileGlob dirname glob

where matchFileGlob is defined in the RebuildMonad module as

matchFileGlob root glob = do
    monitorFiles [monitorFileGlob glob]
    liftIO $ Glob.matchFileGlob root glob

This usage is wrong because the root used to match the glob is not the
same as the root that will be used later when checking the file monitor
for changes. You can see this is suspicious because the declaration of
the monitor does not take an root dir paramater but the immediate
matching does. That's because the root for the monitors is specified
when we do the rerunIfChanged to check the monitor.

So the only correct usage involves passing in the correct root. This is
a ripe source of bugs. So this refactoring moves the root into the
Rebuild monad directly, so the example becomes:

  matches <- matchFileGlob glob

The root is implicit, so you can't accidentally pick a different root
for the immediate match vs the later monitor check. Of course the root
still matters, but if you get that wrong you'll notice immediately
because you will not get the match results you were expecting.

So the root is now passed in with runRebuild, not with rerunIfChanged.

Also change the incorrect use of matchFileGlob. This use case now
relies on the adjusted representation of glob roots, using
FilePath.splitDrive to obtain the root (if any).