Fix command name lookup for known externals

[kubouch]

Description

When known externals were imported from modules with a prefix (use spam instead of use spam *), it was impossible to call them. This PR fixes it.

Fixes #7822

The following now works:

> module cargo { export extern check [] }
> use cargo
> cargo check

User-Facing Changes

Enables subcommand folding of modules with export externs. Should not break anything.

Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo test --workspace to check that all tests pass

After Submitting

If your PR had any user-facing changes, update the documentation after the PR is merged, if necessary. This will help us keep the docs up to date.