Support package fields

Generate proper mappings for package.json fields documented in https://nodejs.org/api/packages.html#packages_node_js_package_json_field_definitions.

• Add mappings for package imports (https://nodejs.org/docs/latest-v15.x/api/packages.html#packages_imports)
• Add mappings self reference (https://nodejs.org/docs/latest-v15.x/api/packages.html#packages_name)
• Follow spec for nested conditions (retry from parent branch as specified in https://nodejs.org/docs/latest-v15.x/api/packages.html#packages_nested_conditions)