Support datapackage.json provided by a user?

[roll]

Overview

The initial implementation doesn't take into account if a user provided a custom datapackage.json.

Depending on the feedback and actual use cases, we need to decide what behavior is optimal:

• the Data Package endpoint is always autogenerated and custom datapackage.json should be accessed directly from uploaded files
• custom datapackage.json replaces the autogenerated one

Another consideration is that the Data Package default endpoint is only capable to reference resources by URL because it's in not the same "virtual plain folder" (in-general, CKAN doesn't have a plain resource folder compared to Zenodo) -- it means the implementation needs to handle path rewriting.