Limit phoc dependencies
Phoc currently depends on mutter, although the dependency is not directly specified (usually phoc is installed by phosh and mutter is pulled in by phosh -> gnome-session -> gnome-shell -> mutter), so in consequence phoc is broken if mutter is missing.
However, phoc does not need from mutter the whole binary, but just the gsettings schemas. In this MR I have implemented a similar solution to debian[1][2], where I just packed the schemas in their own subpackage that phoc can depend on.
[1] https://gitlab.gnome.org/World/Phosh/phoc/-/blob/master/debian/control#L39