Prevent rsc_rc and rsc_cl being installed simultaneously

[rudolfbyker]

Migrated from https://github.com/refstudycentre/misc/issues/2

Implement hook_requirements in rsc_rc and rsc_cl to check that rsc_cl and rsc_rc is NOT installed, respectively. I.e. create an anti-dependency.

Bonus: Find a solution that works for rsc_N modules, where N is the name of any of a number of websites (not just 2).