This adds an API PresetDefinition.from_pkg_resources(). It works both for file-based (no pip install) and module-based preset loads.
I moved over the airline demo presets in this diff to give a concrete example, but once landed should follow up and move over all dagster_examples module presets.
Test Plan: unit
Reviewers: sashank, prha, max
Reviewed By: max
Differential Revision: https://dagster.phacility.com/D2443