Baseline: 5149408a6c2500cb1701880575c41640d4fd1c2f

Cherry picks:

   + b371a986bdb60caf2bb07da0732e8870f2f81ef5:
     Properly account for StarlarkOptions at their default (=null)
     when calculating ST-hash

Important changes:

  - Treat py_*.srcs_version="PY2" the same as "PY2ONLY".

This release contains contributions from many people at Google, as well as Andrew Katson, Benedek Thaler, Brandon Jacklyn, Keith Smiley.