Greboid is an idiot, fix backwards logic.

Change-Id: I2d8ab3313586200fea8d297f21e1b58423a06ca9
Reviewed-on: http://gerrit.dmdirc.com/3373
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Chris Smith <chris@dmdirc.com>
Use the right method when checking bundled plugins

Certain PluginManager methods will read plugin info from disk,
while others just use cached info. There's no obvious way to
tell which is which, and we ended up calling the wrong one.

Now we actually know about plugins that're installed, we can
extract the bundled ones. Yay.

Change-Id: I995f0a5bec8ef06db887127119d4f0f9209bf4bf
Fixes-Issue: CLIENT-498
Reviewed-on: http://gerrit.dmdirc.com/3374
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager