SwiftPM should have a --no-network flag that ensures no network calls are made

SwiftPM should have a --no-network flag that ensures no network calls are made.

When SwiftPM would hit a case of needing to use the network, but this flag would be specified, SwiftPM would fail and describe what it needed to do.

This is important for testing scenarios when the package resolution and build are happening separately and in different environments.