fix: refactor code so most things work without sqlalchemy

also refactor tests to help ensure that remains true

fix: change how we override default app handler, engine maker

per upstream changes. this ensures we avoid the app handler when first
creating the engines, so that config extensions may override the
default app handler without being ignored.

fix: remove duplicated / unused code for `rattail.db.config`

also remove imported names from `rattail.db.util`

Incorporate the WuttJamaican package

- our AppHandler is now based on that one
- our Config *may* be based on it, if so configured

at some point our Config will "always" be based on WuttaConfig but we
need to test a bit more first

Move tests out of the installed package set

this seems to be better practice. from what i can tell this is a
clean move with no breakage.. and built package files no longer seem
to include test modules