Refactor DependencyResolverChain's update logic

Currently the way DependencyResolverChain determines whether a dependency needs to be updated is not very logical (though it works).

Add support for directly invoking a DependencyResolver by name and update the version checking process to use the stored dep_settings.json file to directly figure out which dep resolver to use and call it specifically.

Also consider edge cases when the default resolver chain changes between calls to iotile depends update