gitlab-ci: Extend filtering by package to all retrieval jobs

Currently `FILTER` only works for GitLab projects, the OBS and APT jobs
always retrieve everything, which is particularly costly for OBS as it
scans the content of each package.

Drop the `FILTER` variable and replace it with a `PROJECTS_NAMESPACE`
variable for GitLab and a `FILTER_PACKAGES` variable to be used by
all jobs to narrow the amount of data retrieved, which is often very
useful for testing.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>