When constructing an output profile using --profile-from, it is useful
to be able to override the top level container profile.
Expose a --container-profile option that applies as an override after
other methods for constructing an output profile have run. If no other
method was used, this will result in an empty top level container.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3977>
... in favour of dep.get_variable('foo', ..) which in some
cases allows for further cleanups in future since we can
extract variables from pkg-config dependencies as well as
internal dependencies using this mechanism.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1183>
