Don't recompile nested deps too frequently

When compiling a package with a nested dependency, any modification to the outer
package would trigger a recompilation of the inner package. This commit alters
the fingerprint() method to take a PackageId to query about the location of a
package and only lookup the files relevant to that package.

The dependency structure of a PathSource is now everything rooted at the
original Cargo.toml minus all subdirectories which contain a Cargo.toml
Merge pull request #64 from alexcrichton/no-recompile-nested-deps

Don't recompile nested deps too frequently