Add support for optionally fetching resolved dependencies of projects

[jspahrsummers]

Knowing the resolved dependencies of a particular project can help select better versions during resolution of the top-level project.

For example, given this dependency list:

A == 1.0
B == 2.0

Imagine that A's unresolved dependency list looks like:

C ~> 1.0

and B's unresolved dependency list looks like:

C >= 1.0

Now, if A@1.0 and B@2.0 have already resolved to C@1.0 (e.g., that's committed in a lockfile in each repository), that's probably the best version to use even if there's a C@1.1 available, because 1.0 is the only version known to be tested with both A and B.

If we still wanted it to be parameterizable (in this example, to allow selecting C@1.1 despite the above), we could add a flag or a callback to the consumer.