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 GitoxideLabs#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 (GitoxideLabs#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` (GitoxideLabs#1929).

Author: EliahKagan

gix-packetline-blocking/Cargo.toml

@@ -21,9 +21,12 @@ default = ["blocking-io"]
 ## If set, all IO will become blocking. The same types will be used preventing side-by-side usage of blocking and non-blocking IO.
 blocking-io = []
 
-## DO NOT USE, use instead `gix-packetline` directly.
+## DO NOT USE, instead use `gix-packetline` directly.
 async-io = []
 
+## DO NOT USE, instead use `gix-packetline` directly (and still don't specify `futures-lite`).
+futures-lite = []
+
 #! ### Other
 ## Data structures implement `serde::Serialize` and `serde::Deserialize`.
 serde = ["dep:serde", "bstr/serde"]