Move the dependecy regex instance to the manager instance

We used to do this as a global to save a tiny amount of memory, but this is
problematic when don't call gplugin_manager_init which is private.

I ran across this as I'm porting the unit tests to stop using the default
manager and instead create an instance to help their reproducibility.

Testing Done:
Ran with the turtles.

Reviewed at https://reviews.imfreedom.org/r/2985/