Store content of package.order in JSON output

[AntoineGautier]

Currently, the data from the package.order files are not included in the JSON output.
This ticket is to add this functionality, using for instance an optional key order (array of strings) added to each package.json.
Note that package.order is not a valid JSON in its original format. It is actually not covered by the grammar (it does not follow Modelica syntax).