Prepare for per-target fingerprints

This commit refactors all related infrastructure for the cargo_rustc module to
move the granularity of fingerprints from packages to targets. This involved
heavily modifying the `JobQueue` structure to understand a finer-grained target
for dirtiness propagation and dependency management, and then dealing with
fallout in the main module.

The fingerprint module has been refactored to support a per-target fingerprint.
A fallout of this change is that each build step has its own fingerprint,
including the custom build command step. This will be implemented in a future
commit.

As fallout of this reorganization, we are now exploiting the maximal parallelism
within packages themselves whereas before we were only partially parallelizing.
No new features were added as part of this commit, so I just ensured that all
the tests ran.
Implement per-target fingerprints

This commit refines the granularity of fingerprints from packages to targets,
building on the existing infrastructure. All invocations of rustc now include
the --dep-info flag which will emit makefile-like dependency information. This
information is then parsed later on to determine if all the files are fresh.

The purpose of this commit is to refine the set of files which indicate that a
target needs to be re-built. Before this commit if you modified a test it would
rebuild the entire library, but after this commit it only rebuilds the relevant
test.

Closes #214
Closes #289