Offline mode

[jayvdb]

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.

[indygreg]

Using Cargo in offline is a legitimate feature request. We should add an --offline flag to the various pyoxidizer commands that invoke cargo.

[fabioz]

Ideally, I'd also like to have a way to work offline for pip.

i.e.: I have something as:

    for resource in exe.pip_install([...]):
        resource.add_location = "in-memory"
        exe.add_python_resource(resource)

    exe.add_python_resources(exe.read_package_root(
        path=r'local/folder',
        packages=["local.dep.package"],
    ))

After the initial install, I'm just iterating on fixing things in the local sources, but it seems it needs to do the whole pip thing again and again (which makes things much slower than it needs to be).