Refactor: libpacemaker: Split colocations_list into two functions.

The point of this is to make it easier to follow along with what these
functions do by eliminating some of the conditionals. However, the
names are confusing and probably can't be made much less confusing.

• rscs_colocated_with_list outputs a list of all the resources that are colocated with the given resource argument.
• rsc_is_colocated_with_list outputs a list of all the resources that the given resource argument is colocated with.