Replace engine attributs with target attributes.

Core engine and SQLite tests now passing. The requirement that a target object
be passed to the engine means things need to be fixed in a lot of places, so
expect failures for the next few days as I work though it all.

Also:

* Change `--db-client` to `--client`.
* Make Sqitch a weak ref in Engine.
* Fix a typo in the localized duplicate project URI error message.