The MultiSelectSearcher cannot be used from Plugins 2

The MultiSelectSearcher cannot be created using plugins 2.0. It has a dependency on:

1. ComponentLocator
2. ComponentFactory
   which we do no expose in plugins 2.0.

The workaround is to implement the plugin as a plugins 1.0 plugin.

The custom field plugin documentation suggests that people should be able to reuse our searchers for their own custom field types. We may need to check that all our searchers are available via plugin 2.0.