Inject update manager dependencies nicely.

Move Updater-based providers into a separate module, and inject
the actual dependencies into the DMDircUpdateManager instead of
various 2nd degree dependencies so it can make them itself.

This should make it easier to add other strategies/download
sites/etc.

Change-Id: Id8a7871fbf7d383b05c5fd08e31dea3bd3703444
Reviewed-on: http://gerrit.dmdirc.com/2932
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>