Fix: Global plugins are overwriting User-plugins.

Since the global plugins (from picard/plugins) have been read *after*
the ones in user-plugindir, plugins got overwritten if they have the same
name. Now the user-plugindir is read after the globel one, thus the
user plugins are overwriting the global plugins -- which is more
appropriate.

Signed-off-by: brainz34 <brainz34@musicbrainz.org>