remove outer (pants_exe) lock and serialized cmd

All subcalsses of command (eg GoalRunner and SetupPy) do not require the outer lock
so only the Context _lock is ever actually locked now. Simplified pants_exe and the
number of places we use dirutil.Lock
Replace dep on commons dirutil Lock

Use lockfile, with a thin wrapper to print the owner process commandline
and break the lock if the owner process doesn't exist.