Fix #288: allow mixing --local with and without paths when generating

[davidaf3]

This PR fixes https://github.com/ndmitchell/hoogle/issues/288, allowing to mix the --local parameter with and without paths when generating a database, e.g. hoogle generate --local --local=foo. This way you can include both your dependencies and your own code in a single database. Previously, --local was ignored if another --local=foo was present.

This is not as flexible as the design proposed in https://github.com/ndmitchell/hoogle/blob/master/src/Action/Generate.hs#L40, but it solves the issue for now.