An iterator over OptionValueContainer keys.
Review Request #2472 - Created July 13, 2015 and submitted
Yields exactly one key per option. If multiple keys point to the same value (because the underlying option was registered with multiple names), only yields one of them. Also fixes a minor bug with inverse dest mapping computation. That computation must properly be done only after all options have been registered, so it's not confounded by options on inner scopes that shadow names in outer scopes.
CI in flight here: https://travis-ci.org/pantsbuild/pants/builds/70813876