Fix issue with static methods in class autoloader

[darylldoyle]

Description of the Change

In #158 we added a way to auto-initialise and load classes in the MU plugin and theme.
Because the classfinder uses static methods, it only works on a single application root.

To get around this, we need to manually set the application root before we look for classes in a namespace, so that it can find the right composer.json file

This has been tested on a client project that's using this version of the scaffold.

How to test the Change

• Add a class to the MU plugin and another to the theme.
• Make sure that both are registered.

Changelog Entry

• Fixed - Issue with class loading conflict.

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[darylldoyle]

@joesnellpdx @tlovett1 wondering if one of you could review this since it fixes a bug that could cause engineers some pain in the scaffold.

[fabiankaegy]

Tested this on a project and it works great (y)