Investigate using cargo metadata internally

[acmcarther]

I wrote most of the integration before cargo metadata existed. I'm content with just continuting to use Cargo's internals in the event that cargo metadata is missing something needed, but if its possible to use it today, we probably should.

Crosslinks:
rust-lang/cargo#3815

[acmcarther]

I've rewritten the planner to use output from cargo metadata, but I'm blocked by a small feature request to Cargo. rust-lang/cargo#5122 adds the extra data I need.

Once this change is in, the whole stack will get much more testable as it won't depend on weird internal state of Cargo objects.

EDIT: Prototype here https://github.com/google/cargo-raze/compare/use-cargo-metadata

[acmcarther]

cc rust-lang/cargo#5583

I wasn't aware that these were outstanding issues. I'm currently working around lack of platform-specific details by performing dependency pruning:

cargo-raze/impl/src/planning.rs

Lines 248 to 257 in 2e4128f

	if dep.target.is_some() {
	// UNWRAP: Safe from above check
	let target_str = dep.target.as_ref().unwrap();
	let platform = try!(Platform::from_str(target_str));
	// Skip this dep if it doesn't match our platform attributes
	if !platform.matches(&settings.target, Some(&platform_attrs)) {
	continue;
	}
	}

(using dependency.target and comparing with the configured target).

#53 tracks feature pruning (which is in an unclear state cargo-side as well).

[UebelAndre]

Is this investigation still open?