The regex to find labels inside of angle brackets was not detecting
anything before so I missed all of those.
* Fixed the regex in find-plugin-refs to take care of that.
* Fixed plugin_formatter to emit namespaced labels for the M() macro.
We're porting autogenerated plugin docs so that :ref: targets are
namespaced by plugin type. To do this effectively, we need to know
where all the :ref: targets which refer to the bare plugin names are in
the rst files. This script will find those.
