Since we are adding one more proc macro crate (`zerocopy-derive`), we are refactoring their handling. Thus, instead of using `libmacros_extension` as the common variable to hold the extension for all of them, use a dedicated variable with a more generic name (including for its implementation). Link: https://patch.msgid.link/20260608141439.182634-6-ojeda@kernel.org Signed-off-by: Miguel Ojeda <ojeda@kernel.org>