Compiler needs to be ok with multiple matching crates when using multiple workspaces in RUST_PATH or such

[z0w0]

e.g. cargo install -g tnetstring and cargo install tnetstring. If I was compiling my own project and it's dependencies, I would assume that the tnetstring installed locally would be used (as I might have some sort of custom tnetstring from an unofficial source), but instead it prints out "multiple crates found" like so:

./mongrel2.rc:8:0: 8:15 error: multiple matching crates for `tnetstring`
./mongrel2.rc:8 use tnetstring;
                ^~~~~~~~~~~~~~~
note: candidates:
note: path: /home/zack/coding/hydrox/.cargo/lib/libtnetstring-bfa24df4626b48a4-0.2pre.so
note: meta: #[link(name = "tnetstring",
       vers = "0.2pre",
       uuid = "ce93b70c-c22a-45fa-97a7-66ab97009005")]
note: path: /home/zack/.cargo/lib/libtnetstring-bfa24df4626b48a4-0.2pre.so
note: meta: #[link(name = "tnetstring",
       vers = "0.2pre",
       uuid = "ce93b70c-c22a-45fa-97a7-66ab97009005")]
error: aborting due to previous error

I am working on fixing this with #2662 but I thought I would note the issue regardless.

[z0w0]

Just a note: the same issue is present with Rustpkg so I won't close this, but normally rustpkg wouldn't have an issue because crates are only stored in ~/.rustpkg/lib but there is also going to be a "local workspace" thing being added to rustpkg which will mean local dependencies will be built into .rustpkg/lib and then installed later with the project, so this error will probably be hit again.

[pcwalton]

I don't believe this is backwards incompatible, renominating.

[graydon]

absorbed into #5682