(buildkite 8/n) Make buildkite Python scripts a package


(buildkite 8/n) Make buildkite Python scripts a package

Rather than the hodgepodge of scripts we had under .buildkite, this diff makes those into an installable Python package so that we don't need to do sketchy sys.path manipulation anyore.

After this lands, will need to change the Buildkite pipelines to run the new entrypoint:

  - command:
    - "python3 -m pip install --user -e .buildkite/dagster-buildkite"
    - "~/.local/bin/dagster-buildkite | buildkite-agent pipeline upload"
    label: ":pipeline:"

Depends on D5771

Test Plan: buildkite - see: https://buildkite.com/dagster/test-buildkite-scripting/builds/17

Reviewers: alangenfeld, dgibson

Reviewed By: dgibson

Differential Revision: https://dagster.phacility.com/D5777