Improve dead plugin cleanup
Improve dead plugin cleanup

This isn't a functional change, but makes one of the hairier functions
in PluginManager a bit more readable.

fuchsia-mirror/third_party-xi-editor,google/xi-editor,fuchsia-mirror/third_party-xi-editor,modelorganism/xi-editor,modelorganism/xi-editor,google/xi-editor,modelorganism/xi-editor,google/xi-editor,fuchsia-mirror/third_party-xi-editor,google/xi-editor,fuchsia-mirror/third_party-xi-editor