`cargo check` compiles library object code if a binary exists

[alexcrichton]

Looks like if there's a binary target then the libraries are compiled entirely instead of only requiring metadata.

I ran cargo new foo followed by touch src/main.rs, and also added a dependency on libc from crates.io. After that a cargo check -v yielded:

     Running `rustc --crate-name libc /.../src/lib.rs --crate-type metadata `
     Running `rustc --crate-name libc /.../src/lib.rs --crate-type lib `
     Running `rustc --crate-name foo src/lib.rs --crate-type metadata `
     Running `rustc --crate-name foo src/lib.rs --crate-type lib ...`
     Running `rustc --crate-name foo src/main.rs --crate-type metadata ...`

[alexcrichton]

@nrc could you help look into this?