[dagit] Move dagit/src -> dagit/packages/core/src
This diff moves basically all of dagit/src to dagit/packages/core/src, as part of the larger change breaking up dagit.
This diff is not expected to pass the build, and will be landed along with the other relevant changes.
Test Plan: None.
Reviewers: bengotow, dgibson, max, prha
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D7172