Turns relative imports into absolute imports, and removes imports in init (for generated project skeleton).
When generating a new project with dagster new-repo, the boilerplate code will use absolute imports (rooted at the Python package) instead of relative imports.
Also, the generated __init__.py files will no longer contain re-imports. This should make it easier to rename things in the boilerplate.
Test Plan: local testing and buildkite
Reviewers: catherinewu, rexledesma, sidkmenon
Reviewed By: sidkmenon
Differential Revision: https://dagster.phacility.com/D6799