Previously, we were checking the package with a hard-coded root
directory of ".". This was not a problem before, but with #5372 we
have started to expand globs while checking packages, which breaks if
the CWD is not the directory containing the `.cabal` file and causes
snowleopard/hadrian#634.

Luckily, this is an easy fix: the correct directory is easy to
determine. Writing a test and making sure it's tickling the failing
case took longer than writing the fix!

"." is hard-coded as the root directory passed to `checkPackageFiles`
in a few other places, but those are (a) non-trivial to test, and (b)
already in places that have other assumptions about their CWD, so I
have simply documented the CWD requirement for those.