Define a 'plugin' function to use in place of 'project' that
will figure out where the plugin is living in the root gradle
project.

This allows the plugins to be included and built properly
when they are in a submodule of either the client or the meta
project.

Change-Id: Iacc3551ac9c0bf2343151eb9cf85296c85c5a28b
Reviewed-on: http://gerrit.dmdirc.com/4028
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>