[dagit] Back to relative paths
Summary: It'll be a lot easier to deal with builds by switching our import paths back to relative instead of aliased. Since everyone probably has automatic imports set up in VSCode anyway, the drawbacks of relative paths are mitigated.
Test Plan: Load dagit to sanity check. Buildkite.
Reviewers: bengotow, sashank, prha
Reviewed By: prha
Differential Revision: https://dagster.phacility.com/D7079