Crate paths specified with --extern cause bogus duplicate crates when using symlinks

[dotdash]

Build servo in a path that has a symlink in it currently fails, because the url crate is used in http as well as servo's net. Both are build with the crate for url specified using --extern, but the assigned paths end up differently:

DEBUG:rustc::metadata::creader:   name: url
DEBUG:rustc::metadata::creader:   cnum: 35
DEBUG:rustc::metadata::creader:   hash: 9b74b61cf9c6fb6a
DEBUG:rustc::metadata::creader:    rlib: /home/doener/src/servo/build/x86_64-unknown-linux-gnu/src/support/url/rust-url/liburl.rlib

DEBUG:rustc::metadata::creader:   name: url
DEBUG:rustc::metadata::creader:   cnum: 20
DEBUG:rustc::metadata::creader:   hash: 9b74b61cf9c6fb6a
DEBUG:rustc::metadata::creader:    rlib: /home/doener/storage/builds/servo/x86_64-unknown-linux-gnu/src/support/url/rust-url/liburl.rlib

One path has the symlink (the build dir in this case) resolved, the other doesn't. Looks like there's a call to realpath() missing from existing_match().

Unless somebody beats me to it, I'll create a PR later today or tomorrow.