Fix data-driven-dependencies example app

[captbaritone]

Summary

• Remove hardcoded GRAPHQL_ENDPOINT env var and instead construct the URL from PORT, falling back to 3000
• Add webpack alias for react and react-dom to resolve duplicate React instances when using linked packages
• Import path module in next.config.js for the alias resolution

Test plan

• Run yarn install and yarn dev in data-driven-dependencies/
• Verify the app starts and loads correctly on the default port
• Verify setting a custom PORT env var is respected