Fix the search paths for macro-expanded non-inline modules

[jseyfried]

This PR changes the search paths for macro-expanded non-inline modules so that they match ordinary non-inline modules (fixes #31624). This is a [breaking-change].

Right now, the search paths for a macro-expanded non-inline module are computed as if the module were declared in the top level of the file in which the macro was defined.
For example, consider ./foo/mod.rs:

#[macro_use]
mod inconsequential { // moving the macro outside this module wouldn't change anything
    macro_rules! mod_decl {
        ($i:ident) => { mod $i; }
    }
}

and ./lib.rs:

#[macro_use]
mod foo;

mod bar {
    mod_decl!(baz);
    //^ Now, rustc expects `./foo/baz.rs` (or `./foo/baz/mod.rs`)
    //| After this PR, rustc will expect `./bar/baz.rs` (or `./bar/baz/mod.rs`)
}

r? @alexcrichton

[jseyfried]

cc @nikomatsakis @Kimundi @nrc

[alexcrichton]

I ran this through crater and it reported no regressions. Implementation also looks good to me!

Assigning to @nikomatsakis for a second opinion as well (also to ensure this is expected).

[nikomatsakis]

@bors r+

[bors]

📌 Commit e2aa90e has been approved by nikomatsakis

[bors]

⌛ Testing commit e2aa90e with merge 5b5e521...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt