Implement soft and hard clean

As more bits are included in a project, the amount of files that need to be compiled grow significantly. A soft clean should avoid cleaning the compiled bits, so that cpm would only compile the actual project files. A hard clean would clean also the compiled bits.

The issue names no files or tests. Start by locating cpm's existing clean entry point and how it identifies project files versus compiled bits. Done means a soft clean preserves compiled bits while a hard clean removes them, with both behaviors verified for a project containing dependencies.

Written by the indexing model from the issue text.