Cargo is unable to resolve link conflicts for compiler extensions

[ishitatsuyuki]

When you use a crate that is also used inside rustc...

   Compiling rls v0.1.0 (file:///home/ishitatsuyuki/Documents/rls)
error[E0523]: found two different crates with name `bitflags` that are not distinguished by differing `-C metadata`. This will result in symbol conflicts between the two.
  --> src/build.rs:12:1
   |
12 | extern crate rustc;
   | ^^^^^^^^^^^^^^^^^^^

error: Could not compile `rls`.

To learn more, run the command again with --verbose.

Trigger conditions:

• opt_level is set to 2 (upstream settings)
• Using rustc_private with one of the crates that compiler also use (e.g. bitflags, rustc-serialize)

This is really... broken. This bug is discovered during the process of rust-lang/rust#41639. Most compiler plugins (I tried clippy) are not affected, due to the nature of minimal dependencies they're using. It can be also reproduced on them if I added the dependency and extern crate clause.