Make association names not retain plugins.

Creating associations in plugins should not retain aliases like
'Photos.Photos'. Instead the plugin segment should be dropped and the
association should just be called 'Photos'. This keeps behavior
consistent with previous releases and avoids annoying problems when
trying to contain() association names with `.` in them.
Make association property() exclude plugin names.

When creating association names that include plugin names we should omit
the plugin name from the hydrated dataset. Instead of 'contacts.users',
we should just get 'users' as an example.
Merge pull request #2648 from cakephp/3.0-plugin-associations

3.0 - fix plugin associations