python: cleaning doesn't remove "old" API directories

[jskeet]

If we remove an API path from the config, the now-more-targeted clean operation won't remove the generated code. (It'll remove samples and tests, but not docs and production code.)

Removing a proto from an API should be fine - this is a matter of detecting which directories have previously been generated.

[jskeet]

Idea: find all the gapic_version.py files, and assume that any directory containing that is a GAPIC directory. (Then see if there's an equivalent for docs.)

[jskeet]

Leaving this open to at least consider, although we might want to move it to post-stabilization. (Removing an API path is a very rare thing, and it would be somewhat reasonable to have a playbook which says that the code should be removed as well.)