Refine dependencies on dev-deps

Currently whenever a dev-dep is brought in to the build process the entire
library is rebuilt, but this is just unnecessary recompilation because the
library *can't* depend on the dev-dep.

This commit refines the dependency graph so the lib stage only depends on
transitive dependencies (non-dev-deps), and a new stage for tests was added
which depends on the packages libraries *and* the dev-deps. This way only the
test are rebuilt when dev-deps change, not libraries.
Allow "cycles" through dev-deps

Development dependencies can never be the root of a cycle because nothing
depends on a development dependency, so there's no need to track the start of a
cycle at the edge going out to a development dependency.

If a cycle is later detected, it will still be reported.
Fix the dependency graph with root pkg cycles

If the root package ended up depending on itself through some development
dependency (technically not a cycle), then the resolve phase would currently
overwrite some previous result, destroying the progress. By registering the root
package as seen early on this prevents the overwriting from happening and
instead appending happens.
Fix dependencies linking to the local package

This is a legitimate possibility which doesn't necessarily imply a cycle, so the
case needs to be handled.
Add a test for #432

All problems have been fixed in the previous commits, and this now closes #432
auto merge of #630 : alexcrichton/cargo/issue-432, r=brson

This is a series of commits which culminates in fixing #432, fixing a number of other related issues along the way. The biggest user-facing fix here is that if you run `cargo build` followed by `cargo test` your library will no longer be rebuilt if you have dev-dependencies.
Don't always run doc tests for the root package

When using `cargo test -p`, be sure to run only the doc tests for the package
actually being tested.

Closes #660