rustdoc `[src]` link for modules goes to `mod x;` when the module is re-exported across crates

[chisophugis]

When I click the master [src] link at https://doc.rust-lang.org/std/fmt/ I get taken to https://doc.rust-lang.org/src/alloc/lib.rs.html#178 which is a bit surprising.

I would have expected to be taken to something like https://doc.rust-lang.org/src/alloc/fmt.rs.html#11-567 which is the destination of the [src] link at https://doc.rust-lang.org/alloc/fmt/index.html