Test function imports replaced by module imports.

[rwakulszowa]

Test for #18347.
The issue affects all code using import merging. When a function and a module share the same identifier, e.g.

pub fn foo() {}
pub mod foo { pub fn bar() {} }

rust analyzer often confuses the function with the module, generating invalid code.

[rwakulszowa]

I haven't been able to actually find a proper fix yet. I can't find a reliable way to tell the merging function whether an import is a module (and can be used for merging) or a function.

I'm not sure if I manage to find a solution, so I'm sending a small PR with what I have.

[Veykril]

The IDE layer currently lacks being able to resolve something to multiple namespaces (Semantics::resolve_path only yields one namespace at most), so that needs tackling first and is unfortunately a bigger change.