Fix zsh completion in nested modules

[kitsuyui]

Fixes #2912.

This updates zsh completion so it can continue completing commands inside modules, instead of stopping after the first module segment.

What changed:

• resolve the current module path from the words already typed by the user
• use just --list <module> to complete names at the current module level
• detect fully-qualified recipe paths with just --summary
• add a zsh regression test covering nested modules

Before this change, completion could stop after the first module name in cases like the one described in #2912, where a command should continue completing deeper module recipes.

[casey]

I switched to using Clap's dynamic completion engine in #3167. The new completion engine calls into the just binary to get completions. This allows us to implement custom completion logic in Rust for all supported shells, instead of having to patch in support for each individual shell.

I created #2406 to track improvements to the new completion engine in one place.

The new completion engine will complete :: separated recipe paths. It doesn't work with space-separated paths, although there's an item in #2406 for that.