Improve FixtureManipulator DX: validate package name + ensure StageFixtureManipulator is committed + ensure `package_manager_bypass_composer_stager` is not set to FALSE

Problem/Motivation

Follow-up for #3322913: Create an easy way for functional tests to simulate an update (and update kernel tests to use the same) → .

We saw several people already adopt the test infrastructure that #3322913: Create an easy way for functional tests to simulate an update (and update kernel tests to use the same) → introduced. And in doing so, we spotted 3 problems:

1. passing in a Drupal project name instead of the corresponding composer package name
2. Creating a stage and "beginning" it before StageFixtureManipulator was either created or ready to commit.
3. if package_manager_bypass_composer_stager is set to FALSE, that \Drupal\package_manager_bypass\Beginner will never run, which also means that StageFixtureManipulator will never run.

Proposed resolution

1. Add basic validation.
2. Detect this and inform the developer
3. Detect this and inform the developer

Remaining tasks

User interface changes

API changes

Data model changes