Offline mode

cargo build has an --offline mode. PyOxidizer should also, even if it only works reliably for projects without pip dependencies that need to be fetched. #199 allows existing venvs to be reused without pip fetches if the requirements are already met.

It will fail atm on pip fetches, but there are other issues about improving that, and this is similar to cargo offline failing when new deps are added.

No files or tests are named. Start by tracing PyOxidizer's existing dependency-fetching and virtual-environment reuse behavior, including the path described in #199. Done means an offline option prevents network fetches, works when required dependencies are already available, and fails clearly when they are not.

Written by the indexing model from the issue text.