cam_comp.F90: cleanup module subroutines to other modules

### What is the feature/what would you like to discuss?

During the review of #504, @nusbaume originally wrote:
> I wonder if long-term it would be best to move this subroutine, and cam_register_constituents, out of this module and into other modules (e.g. the ones added in this PR), and then just use them here. This is because I think ideally cam_comp would contain the core CAM-SIMA phases and nothing else.

### Is there anyone in particular you want to be part of this conversation?

@nusbaume @peverwhee

### Will this change (regression test) answers?

No

### Will you be implementing this enhancement yourself?

Any Software Engineer can do this

Start by reading cam_comp.F90 and the modules added during PR #504, focusing on the subroutine discussed in that review and cam_register_constituents. Determine which existing modules should own these routines while keeping cam_comp limited to core CAM-SIMA phases. Confirm that the regression-test answers remain unchanged.

Written by the indexing model from the issue text.