Stage::require() should validate the incoming package names

Problem/Motivation

follow -up to #3304569: Validate package names are in the correct format in Installer

Steps to reproduce

Drupal\package_manager\Stage::require takes package names but we don't actually validate they are in the correct format so you can send project names and they will get saved directly.

Proposed resolution

check it is in the right form namespace/package_name and throw an exception.

We should a new test method in Drupal\Tests\package_manager\Kernel\StageTest

Remaining tasks

• ✅ File an issue about this project
• ☐ Code Review
• ☐ Automated tests needed/written?