Add `dux` command, rename `dedux` to `undux`

[cldellow]

dux should do what the indexer does, in a blocking, synchronous manner. This would be useful for:

• testing, when you want to ensure the indexer has finished before exercising the UI
• immutable deployments of read-only datasettes, where you'd like the (eventual) power-user search to work w/o a background indexer running

[cldellow]

As part of this, we'll fix up the names of the prepare/index functions, and create a cleaner boundary between the bits that expect Datasette's Database module and the bits that expect a sqlite3.Connection.