Hackage requires the .cabal file to be named consistently with the package name,
but `cabal check` didn't detect this yet.

Example output:

    $ cabal check
    The following errors will cause portability problems on other environments:
    * The filename ./doo.cabal does not match package name (expected: foobar.cabal)

    Hackage would reject this package.

Note: this new check is implicitly/accidentally tested by an existing (unrelated)
test in `cabal-testsuite`.