Consider supporting zipfiles for PYTHONPATH dependency analysis

[zackverham]

Follow-up from #6, specifically:

#6 (comment)

[amol-]

We should probably support a list of extra paths to consider part of the system directory on top of the standard one inherited via the venv.

If those path come from PYTHONPATH or other manners can be abstracted from picopip, which would allow picopip to retain the property of being able to act on a target python environment independently from the state of the python interpreter invoking picopip itself

Expanding PYTHONPATH inside picopip was a good immediate fix for systems that were expecting those packages be there, but now that the issue is under control it would make sense to implement support for any target path that code owning the environment knows it's necessary to include for inspection.