Support installing plugin to libraries/ckeditor/plugins

Looking at other CKEditor plugin modules and some of the documentation on drupal.org, it seems to be a defacto standard to have ckeditor plugins in libraries/ckeditor/plugins/pluginname.

This makes sense since it removes the risk of confusion and conflicts with non-ckeditor libraries that might have the same name and CKEditor's default instructions are to put plugins in a plugins sub-directory, which is what Drupal core does with its default CKEditor plugins.

If we can get consistency with where CKEditor plugin modules store their libraries it also makes it easier to manage all CKEditor plugins in a uniform manner.