[LINTER] Enforce source code file name conventions

Summary:
Follow-up for D3572.

This linter ensures the source file names are compliant with the naming
conventions from the developer notes.

Test Plan:

arc lint --everything # No error

Rename src/addrdb.cpp to src/addrDb.cpp

arc lint -- src/addrDb.cpp

Check the error is catched by the linter.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3593