WIP: Add support to build subprojects of a main meson project

commit 043f5ebc6f3e8ed92f60f0d7addfac02fa1f4613

DO NOT MERGE: Temporaly use custom gstreamer branch

commit 0b94bfe3175f7a53faa91b52afa76eb8d410c8d3

ci: Move needed scripts from gst-ci to cerbero

When GStreamer's CI triggers cerbero's CI it sets CI_GSTREAMER_URL and
CI_GSTREAMER_REF_NAME in the env pointing to the repository and branch
to build.

commit c2c3237c

Use unified GStreamer repository

Since all GStreamer recipes now share the same git directory, we have to
use a lock to avoid multiple recipes doing the git checkout
concurrently.

commit fd638991

source: Take recipes_remotes value from config

Config files have recipes_commits and recipes_remotes properties but
only the former was actually used. Move the code handling this into
the base class.