Implementation PR: #4770
Original issue: #4686

Summary:

Running Cargo with -Z offline flag changes the resolution process such that it only considered crates, already downloaded locally.

Steps:

• currently, there's no way to populate local crate storage, see Add an "airplane" flag to Cargo, altering it's dependency resolution behavior #4686 for details

• write precise documentation about effects and intended use-case for this flag. See https://github.com/rust-lang/cargo/blob/5845464cf92a348f95dc6b43133c017c37fcb684/src/doc/src/reference/unstable.md#offline-mode for current docs

Stabilization TODO:

• change -Z offline unstable cli option to a stable one, which works the same way as --frozen and --locked flags do.
• -Z offline actively breaks building crate offline #6014