[repoapi] move get_repository function into a new repoapi module
This new module aims to host the function of the new API replacing the old
DBAPI. `get_repository` is still needed hence moved to the new module.
Related to #2503918
usr/lib/python2.4/site-packages/cubicweb/
etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances