Our metadata extraction used to export a nested dict structure
loosely based on JSON-LD with schema.org/CodeMeta.
This diff makes it output proper JSON-LD data.
Note that because of some issues (eg.
https://github.com/codemeta/codemeta/issues/198 and the impossibility to
represent a software entity without a specific version), I removed
`softwareRequirements` and `softwareSuggestions` from the output
As a side-effect, it resolves T1289
PS: Morane, don't review this diff today, it can wait until tomorrow :)