ctest: Check public API in modules

Currently ctest requires everything on the Rust side to be available at the crate root. We should check nested modules too, though, since we may have more in the future when things conflict.

I think this could be implemented by adding API to ctest like the following:

// Same for `Struct`, `Const`, etc
impl Struct {
    pub fn path() -> ItemPath;
}

pub struct ItemPath(BoxStr);

impl ItemPath {
    pub fn as_str(&self) -> &str {
        &self.0
    }

    pub fn components(&self) -> impl DoubleEndedIterator<Item = &str> {
        self.0.split("::")
    }
}

Cc @mbyx

Start by reading ctest's existing crate-root API checking and inspect how items such as Struct and Const are represented. Trace how paths are currently handled, then verify that nested module paths can be checked and that the proposed ItemPath behavior is covered by tests or equivalent validation.