Wrong path in error messages when compiling a local dependency with a path attribute

[ebfe]

Consider:

$ cargo new foo
$ cd foo
foo $ cargo new bar
foo $ cat << EOF >> Cargo.toml
> [dependencies.bar]
> path = "bar"
> EOF
foo $ echo "this is a syntax error" >> bar/src/lib.rs
foo $ cargo build
   Compiling bar v0.0.1 (file:///tmp/foo)
/tmp/foo/bar/src/lib.rs:4:1: 4:5 error: expected item, found `this`
/tmp/foo/bar/src/lib.rs:4 this is a syntax error
                          ^~~~
[...]

This works as expected. Adding a [lib] section with a path attribute to
bar/Cargo.toml results in errors printed with a path relative to bar, which
confuses editors trying to match errors to source lines.

foo $ cat << EOF >> bar/Cargo.toml
> [lib]
> name = "bar"
> path = "src/lib.rs"
> EOF
foo $ cargo build
   Compiling bar v0.0.1 (file:///tmp/foo)
src/lib.rs:4:1: 4:5 error: expected item, found `this`
src/lib.rs:4 this is a syntax error
             ^~~~
[...]

[alexcrichton]

This is a dupe of #694 essentially