If a plugin gets disabled due to a `disabler()` dependency, the plugin
docs build itself will get disabled because `all_plugins_paths` will
become a disabler.
This was actually happening with opencv on systems that don't have
opencv available, and could happen with libsoup too if the build files
change in the future.
Let's avoid wasting hours of debugging for people. A not-found
dependency has the same effect.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8582>