Instead of always parsing the python stage to load meta information
allow the user of a new `{stage}-meta.json` file. This is a first
step towards allowing modules to be written in a different language
than python. It also has some practical advantages:
- slightly faster as it avoids calling python to output the schemas
- easier to write schemas as this can be done in a real json editor
now
- more extensible in a future where stages maybe binaries with
shlib dependencies that are only satisfied in the buildroot
but not on the host
