Auto merge of #3144 - alexcrichton:less-update-registry, r=brson

Avoid updating registry when adding existing deps

Cargo previously erroneously updated the registry whenever a new dependency was
added on a crate which already exists in the DAG. This commit fixes this
behavior by ensuring that if the new dependency matches a previously locked
version it uses that instead.

This commit involved a bit of refactoring around this logic to be a bit more
clear how the locking and "falling back to the registry" is happening.

Closes #2895