CCPP metadata files are not seen by CAM-SIMA if they are in SourceMods

### What happened?

If one adds a modified CCPP metadata file to `SourceMods/src.cam`, that modified file will simply not be seen by the build system. This is true even for an initial (post-clean) build, or if the relevant source code file is in `SourceMods/src.cam` as well.

The build system (including the build cache) should be able to recognize that a modified metadata file is in SourceMods, and then use that file accordingly when generating the CCPP caps.

### What are the steps to reproduce the bug?

Create any case, add a metadata file with a modified standard name to `SourceMods/src.cam`, run `preview_namelists`, and then see if the associated CCPP-generated cap files in `bld/atm/obj/ccpp` contain the modification you added.

### What CAM-SIMA hash were you using?

7d299b6

### What machine were you running CAM-SIMA on?

CISL machine (e.g. cheyenne)

### What compiler were you using?

GNU

### Path to a case directory, if applicable

_No response_

### Will you be addressing this bug yourself?

Any Software Engineer can do this

### Extra info

To get around this particular issue, one needs to modify the metadata file of interest directly in the local copy of CAM-SIMA, clean the build, and then re-build the model from scratch.