Cargo stopped including optional dependencies into build scripts

[hdevalence]

With rust-toolchain = nightly-2018-04-30, it's possible to have an optional dependency specified in Cargo.toml as, say,

[build-dependencies]
...
serde = { version = "1.0", optional = true }
...

and in build.rs as, say,

#[cfg(feature = "serde")]
extern crate serde;

Then cargo invokes rustc with

rustc --crate-name build_script_build build.rs ... --cfg 'feature="serde"' ... --extern serde=...

However, with rust-toolchain = nightly-2018-05-03, cargo invokes rustc without the --extern serde flag:

rustc --crate-name build_script_build build.rs ... --cfg 'feature="serde"' ... 

This causes a build error, since the crate can't be found any more.

[alexcrichton]

Aha a very subtle bug caused by #5415! I believe this should be fixed by #5480, sorry for the breakage!