Dependency check of the tidy tool should verify libstd dependencies

[gnzlbg]

There is a documentation comment explicitly saying that it is crucial for libcore,liballoc,libstd not to depend on any crate with incompatible licenses, yet a couple of lines after there is an exemption that does precisely this: https://github.com/rust-lang/rust/blob/master/src/tools/tidy/src/deps.rs#L25

AFAICT, this property of the runtime is not verified by the tidy script, but should.