Previously for global option --bar-baz you had to set PANTS_DEFAULT_BAR_BAZ.

Now we allow PANTS_BAR_BAZ, eliding the explicit global scope 'DEFAULT'.
If the option name itself already starts with 'pants-', e.g., --pants-workdir,

we also allow PANTS_WORKDIR, instead of PANTS_PANTS_WORKDIR or

PANTS_DEFAULT_PANTS_WORKDIR.
Using env vars to set option values isn't necessarily common, but this will make

it more conveninent and intuitive. Also, it will allow the PANTS_CONFIG_OVERRIDE

mechanism to be retconned into a --config-override option.

CI passes.