Auto merge of #5468 - matklad:aliases, r=alexcrichton

Tweak handling of aliases

Previously, `execute_subcommand` was called recursively, and each call
would `.configure` the `config` again. It worked, but seemed rather
fragile.

This commit handles aliases more explicitly, ensures that `.configure`
is called once and, as a bonus, adds a warning for when an alias is
shadowed by the built in.