refactor: nest NODE_VERSIONS, version -> repositories

[gzm0]

This is in preparation for facts (#3916) which will need this structure.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

The (generated) NODE_VERSIONS value mirrors the structure of the node_repositories attribute.
As a result, extracting which node versions are built-in requires string mangling.

What is the new behavior?

The NODE_VERSIONS value is keyed by node versions. The values are the "node repositories".

Does this PR introduce a breaking change?

• Yes
• No

Other information