Deps: an offline switch, and fetching ahead of time #228

Part of #227. Proposal: ComlineProject/docs#163, proposals/offline/fetching.md.

• An offline mode for dependency resolution: no network at all, set by the caller (the CLI's --offline, an environment variable, machine config).
• Offline, a build that is missing pinned content fails with one error listing everything missing, and how to supply it. Today it stops at the first.
• An entry point that fetches exactly what a build would fetch and nothing else, for comline fetch.
• Builds on the existing no-fetch mode (FETCHING, compile_package_without_fetching) in core/src/package/deps.rs.