`dbmate` package was starting to get a bit polluted. This PR migrates each driver into a separate package, with clean separation between each.
In addition:
* Drivers are now initialized with a URL, avoiding the need to pass `*url.URL` to every method
* Sqlite supports a cleaner syntax for relative paths
* Driver tests now load their test URL from environment variables
Public API of `dbmate` package has not changed (no changes to `main` package).
