Use the compile_commands implementation from the monolithic rule in the per_file rule

[furtib]

Why:
Currently, we use a different, inferior implementation for generating compile_commands.json in the pre_file rule.
We should use the same as in the monolithic rule. This would prevent us from having to maintain both rules simultaneously.

What:
Replaces the compile_commands.json generating implementation in the pre_file rule.
Fixes the unittest for compile_commands, the per_file compile_commands.json location changed

Addresses:
None