more bot work

  - streamline Makefile,infra_tests.py,gen_tasks.go down to
    just Makefile and gen_tasks.go.
  - add Mac bot
  - add an "skcms" bot that just depends on all the others
  - run bot.py
  - pass a package to bot.py on Linux

Change-Id: I70b53c123134080da27024a6c80369e9d30abbaa
Reviewed-on: https://skia-review.googlesource.com/95021
Reviewed-by: Mike Klein <mtklein@chromium.org>
6 files changed