Make swh db init and swh db-init behavior much closer to one another

While named similarly, these two CLI utilities used to take quite a different
approach to doing the same thing.

They now both call out the same initialization function for a given module. The
main difference is now the source of information for what databases to
initialize: `swh db init` uses the SWH_CONFIG_FILENAME configuration file, while
`swh db-init` only uses its own command line arguments.