Auto merge of #10082 - basile-henry:basile/alias-shadow-external-warning, r=ehuss

Warn when alias shadows external subcommand

As per #10049, we start by emitting a warning when an alias shadows an
existing external subcommand. After a transition period (duration not
specified), we will make this a hard error.

Should the warning mention that this will become a hard error?

diff --cc src/bin/cargo/cli.rs
Simple merge