The msg "perhaps this crate needs to be recompiled?" should include paths

[pnkfelix]

This was a very interesting and potentially useful warning from rustc:

cfg/mod.rs:13:1: 13:20 error: found possibly newer version of crate `std` which `rustc` depends on
cfg/mod.rs:13 extern crate rustc;
              ^~~~~~~~~~~~~~~~~~~
cfg/mod.rs:13:1: 13:20 note: perhaps this crate needs to be recompiled?
cfg/mod.rs:13 extern crate rustc;
              ^~~~~~~~~~~~~~~~~~~

It would be even more useful if it told me the paths it was using for the two crates involved (std and rustc), so that I could be sure that the files I am inspecting (in my file system, and in practice in my makefile debugging) actually correspond to the ones that rustc is informing me about.