Whenever I need to install an app using the package installer, there will be a prompt asking for admin user's name and password. How do I temporarily turn this feature off, so that any package can be installed automatically?
You'll want to go into your System Preferences, and into Security & Privacy
Then unlock - if needed - the lock on bottom left, and select "Anywhere". The computer might ask you for your password.
When you're done, you should be able to open and install many .DMG installers without being prompted for a password.
When you are done, you can revert these settings as they are a good security to prevent third party apps from installing automatically on your computer.