Opening Documentation for module opens parent module

[CryZe]

If you try to open the documentation for a module, it directs you to the parent module instead.

Steps to reproduce:

1. Write something like this use std::mem::swap;
2. Put the cursor on mem
3. Run Rust Analyzer: Open docs under cursor
4. The documentation for std itself and not the mem module opens.

[matklad]

cc @zacps

[zacps]

Cheers for the report, I'll try to take a look at it sometime this week.