Renamed optional dependency should use the renamed name

[withoutboats]

Given a Cargo.toml dependency stated like this:

[dependencies.dalek]
package = "ed25519-dalek"
version = "0.7.0"
optional = true

I expected to be able to turn the dependency on with cargo build --features dalek. However, this does not work. cargo build --features ed25519-dalek does work.

I think we should be consistent here in treating the dependency name, not the package name, as the name of this optional dependency.

[alexcrichton]

I agree yeah that this should use the name dalek for the feature, and this should be fixed in #5811