Rename `dencam.py` module to `fido.py`

[iingram]

We have retronym'd the version of the firmware that runs on the buster-based Raspberry Pi OS to fido and we should at some point post-deployment period, rename the actual python module that runs this firmware to reflect that.

[iingram]

@kgarwoodsdzwa this may be pretty straight-forward but we should probably couple the actual literal renaming step with some commits in the same topic branch that address things like changing all the places in the README that reference dencam.py to fido.py etc.