Add a second "DO NOT USE" feature to `gix-packetline-blocking`

This adds a `futures-lite` feature to `gix-packetline-blocking`. The new feature is undocumented except for the warning not to use it. It does nothing, and its purpose is to support an existing internal use of `gix-packetline-blocking` through an alias named `gix-packetline`. This new feature should never be used except to keep `cargo check` commands with `--workspace` that list `gix-packetline/futures-lite` from failing due to the absent feature. Such `cargo check` commands are rare and `cargo check` should not typically be used this way. But RustRover automatically composes and runs such a command. This fixes a RustRover project discovery breakage commented on in #1929. This "ghost feature" may be removed without warning. Nothing should rely on it in production or otherwise in a significant way. It is a bug for any software to break or change behavior if it is removed. This addition is similar to the addition in be4de0d (#1123) of the `async-io` feature of `gix-packetline-blocking`, which likewise shouldn't be used. However, `gix-packetline-blocking/futures-lite` is even less elegant than `gix-packetline-blocking/async-io`, since `gix-packetline` doesn't explicitly delcare a `futures-lite` feature. Instead, we currrently don't use `dep:` for `futures-lite` because it breaks `cargo-auditable` (#1929).

EKEliah Kagan committed 1 year ago