Explicit packageFormat (or similar) claim for package.json recipes

[AMDphreak]

Dub historically accepted package.json as a recipe filename (same name npm uses). Overlapping keys (name, version, dependencies) make heuristics brittle. Related work already refuses obvious npm files (#3118, #3136) and discusses deprecating the alias (#789).

Proposal

Beyond deny-lists of foreign keys: require (or strongly prefer) an explicit ecosystem claim when loading a file named package.json, for example:

{
  "packageFormat": "dub",
  "name": "my-pkg",
  "targetType": "library"
}

Suggested rules for Dub:

1. Prefer dub.json / dub.sdl (already true).
2. If the recipe file is named package.json and packageFormat is present:
   • "dub" → load as today
   • anything else → refuse with a clear error
3. If packageFormat is absent on package.json: keep today's legacy/heuristic path for a deprecation window, then refuse or ignore (so bare unclaimed package.json stops being treated as Dub by default).

Exact field name is negotiable (packageFormat, ecosystem, etc.). The point is a positive claim, not only "looks like npm."

Cross-ecosystem ask

The same field only helps if JS tooling documents a matching claim ("packageFormat": "npm" or equivalent). Filing this with Dub and with the OpenJS Package Metadata Interoperability group so both sides can pick one key.

Motivation

Mixed monorepo: Antora/npm package.json at repo root, Dub recipe under app/dub.json. Root dub build misread the npm file until heuristics / a root dub.json intervened. An explicit claim would make that class of failure obvious and tool-agnostic.

[AMDphreak]

Cross-posted for coordination:

• OpenJS Package Metadata Interoperability: Explicit packageFormat claim so non-JS tools can disambiguate package.json openjs-foundation/package-metadata-interoperability-working-group#55
• npm RFCs (awareness): [RRFC] Explicit ecosystem claim on package.json for cross-ecosystem disambiguation npm/rfcs#913

[Geod24]

We're just going to drop package.json and rely entirely on dub.json.