Always output valid JSON-LD.

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 #1289 (closed)

PS: Morane, don't review this diff today, it can wait until tomorrow :)

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Migrated from D617 (view on Phabricator)