Move to an in tree mach command

[indygreg]

This tool seems useful. I think it should live in mozilla-central:/tools and be a mach command so the barrier to using it is lower.

[EricRahm]

@indygreg FWIW this is now a python script, not sure what it would take to integrate into mach though.

[indygreg]

Look for various mach_commands.py files in mozilla-central.

A mach command is just a Python function with some decorators defining the command name and arguments. See e.g. python/mozbuild/mozbuild/frontend/mach_commands.py for some good examples. You can even install packages into a virtualenv using pip. See https://gecko.readthedocs.io/en/latest/python/mach/commands.html for more.