fix: resolve directory imports with index.js in v2 addons

[lifeart]

Problem

Two related issues affect v2 addons that use directory structures with index.js files:

1. ESM strict resolution: When a v2 addon has "type": "module" in package.json, webpack 5's strict ESM resolution requires fully specified paths. Directory imports like ./components/my-component fail because webpack won't automatically resolve to ./components/my-component/index.js.

2. Conditional exports with .js pattern: When a v2 addon's package.json has exports like "./*": { "default": "./dist/*.js" }, importing addon/components/dir-component maps to dist/components/dir-component.js which doesn't exist — the actual file is dist/components/dir-component/index.js.

Solution

1. fullySpecified: false webpack rule for .js files — disables strict ESM resolution, allowing webpack to naturally fall back to index.js for directory imports.

2. Index fallback in AutoImportResolverPlugin — when module resolution fails, try appending /index to the specifier. This makes the exports pattern produce dist/components/dir-component/index.js which does exist. This integrates cleanly into the resolver plugin infrastructure from implement renamed-modules in webpack plugin to support use-ember-modules #705.

Changes

• resolver-plugin.ts: Add /index fallback after defaultResolve() fails for package imports
• webpack.ts: Add fullySpecified: false rule for .js files
• v2-addon-test.ts: Add two test addons:
  • addon-with-index: ESM type: module addon with directory/index.js structure
  • addon-conditional-exports: addon with "./*": "./dist/*.js" exports pattern and directory components

Test plan

• All 13 v2-addon test scenarios pass (ember3, release, releaseWithModules, beta, canary, fastboot, shim-requires-auto-import, cross-talk)
• New tests verify ESM directory imports resolve to index.js
• New tests verify conditional exports with .js pattern resolve directory imports via fallback
• Existing flat file imports continue to work with conditional exports

🤖 Generated with Claude Code