chore: Remove externalNames from unplugin

[aleksanderkatan]

Blocked by #2502, #2505
externalNames make transformed function code contain redundancy, since it can always be retrieved via Object.keys from externals.

  const metadata = `{
    v: ${FORMAT_VERSION},
    name: ${name ? `"${name}"` : 'undefined'},
    ast: ${embedJSON(ast)}, // <- redundancy here
    externals: () => ({${ast.externalNames
      .map((e) => (e === 'this' ? '"this": this' : e))
      .join(', ')}}),
  }`;