Make usage of ClassGraphFacade replacable with custom logic in ReflectionUtils

[pokab]

...by providing user injectable Function, while retaining ClassGraphFacade's lazy loading semantics.

Moves concrete subtype cache to ReflectionUtils.

Additionally:

• All tests pass (as I have built a snapshot for myself).
• I didn't create an issue. Should I?

[fmbenhassine]

Thank you for your PR. However, why should we make classgraph replaceable? If I follow this reasoning, we should also find a way to make objenesis replaceable, and datafker replaceable as well.

If you want to customize the way to scan the classpath for concrete types, you can disable the scanClasspathForConcreteTypes parameter and provide a custom object factory. I see no need to add an indirection to make a dependency swappable. Do you agree?

[fmbenhassine]

This will be resolved with #474.

Thank you for your contribution anyway.

[pokab]

My reason behind this change had to do with a requirement in our test stack, where we had to filter classes on the classpath. The solution in #474 (which is not yet merged) should provide the same possibility. Thank you.