Always output valid JSON-LD.

Summary:
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 :)

Reviewers: moranegg

Reviewed By: moranegg

Subscribers: swh-public-ci, zack

Maniphest Tasks: T1289

Differential Revision: https://forge.softwareheritage.org/D617