Rename modules in 'relengapi' to be in 'relengapi.lib'

Just for consistency, and to leave the 'relengapi' namespace clean

* [x] `relengapi.celery`
* [x] `relengapi.db`
* [x] `relengapi.proxy`
* [x] `relengapi.subcommands`
* [x] `relengapi.testing`