i mean, exchange the behaviour of the caps lock key with that of the control key.
ucontrol, and fkeys let you do that. both seem to be unmaitained though.
The Keyboard prefpane trivially lets you map the Caps Lock key to Control. I used that setup for a long time. Are you actually asking to keep Caps Lock around but map it to Control? I guess that really is tricky, but it never occurred to me as useful, because I never use Caps Lock, so I just happily have two Control keys.