Clean up byte-compilation code in packaging (#11254 followup).

Clean up byte-compilation code in packaging (#11254 followup).

- Don't use keyword arguments for debug_override; I find it more
  readable to have a comment explaining that True makes pyc and False
  pyo than to write out the non-obvious (when you haven’t read the doc)
  argument name

- Move duplicate code from build_py and install_lib into cmd

- Remove obsolete verbose argument of util.byte_compile

- Remove obsolete passing of -O/-OO to the Python process spawned by
  util.byte_compile (I’ll remove the whole spawning later, after I write
  more tests to check the contents of pyc and pyo files; now that
  byte_compile does not depend on the value of __debug__ in the calling
  Python, we can call py_compile or compileall directly)