Introduce a per-charm configuration file `.sunbeam-build.yaml`,
containing information about needed external libraries, internal
libraries, and configuration templates.
Introduce new management script `repository.py`, responsible for
preparing, cleaning and updating libs.
Move all internal libraries back to their original charms. Each charm is
responsible of its own libraries.
Change-Id: I9edabed1c252cae60fcd945b931952aeaef12481
Signed-off-by: Guillaume Boutry <guillaume.boutry@canonical.com>