Deprecate PluginManager.getDirectory().

This is injectable - you shouldn't need a PluginManager to know
where the files are.

Also give CorePluginExtractor an injectable ctor instead of having
a useless provides method.

Change-Id: Id96f671dbd55a5ad8f130f45d4d2eb7ca3d2c9ab
Reviewed-on: http://gerrit.dmdirc.com/3649
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>