Rename pot2json.py -> po2json.py

The "pot2json" name was misleading; this script is really intended for
use *after* translations have been fleshed out (e.g. when you have a
LANGCODE.po file, for any value of LANGCODE) rather than when you have
an as-yet-untranslated app.pot file. (If the .pot file should be
converted to some other format, that might as well happen inside the
extract-translation-ids.py script, as the .pot file is being created.)