Avoid double registration of commands
Avoid double registration of commands

When a given subcommand is already found:
- if it is a command namespace, we replace it with the new command, and attach all children of the namespace to the new command.
- if it is a regular command, we skip registration, to avoid double registration. This means only the first registration counts (making overrides through packages and requires precede bundled commands).

Fixes #5186

wp-cli/wp-cli,wp-cli/wp-cli