Add support for database flavors to `swh db-init`

This adds a "flavor initialization step" to the initialization of the database, after the execution of the sql file named *-flavor.sql.

I've added a few sanity checks to warn if one of the underlying assumptions for this feature breaks.

Test Plan

called swh db-init with a bunch of combinations of valid and invalid flavors to check the behavior.

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Migrated from D3997 (view on Phabricator)