Reverting 61f325c687592225bc24698585633380e52798fd so that BuildTrigger matches the name of its core class.
Using the Descriptor.clazz.name rather than Descriptor.displayName in @Describable did not work.
The HTML for the selection contained as expected:
  <div descriptorId="hudson.tasks.BuildTrigger" name="publisher" title="Build other projects">
  <input name="stapler-class" type="hidden" value="hudson.tasks.BuildTrigger"/>
Yet Control.findDropDownMenuItem does not find this because it is checking by.link(caption)
and the actual link seems totally disassociated, containing only:
  <li index="2" groupindex="0" id="yui-gen34" class="yuimenuitem"><a class="yuimenuitemlabel" href="#">Build other projects</a></li>