Baseline: 61a5fae1b49ed067350d93578da268715031df06

Important changes:

  - The deprecated "relative_to_caller_repository" parameter has been
    removed from the Label constructor.
  - The toolchain transition is now enabled for all toolchains.

This release contains contributions from many people at Google, as well as Artem V. Navrotskiy, hvadehra, Ikko Ashimine, Paul Gschwendtner, ron-stripe, Yuval K.