Introduce `Resource.of()` and make `DefaultResource` package-private

[sbrannen]

As I mentioned in #4876, DefaultResource does not need to be a public type.

It should rather be package-private, and we should introduce a Resource from(String name, URI uri) static factory method in Resource that allows one to create a default resource.

Furthermore, since DefaultResource was previously documented as @API(status = INTERNAL) with a prominent WARNING in the Javadoc, making it package-private in JUnit 6.0 should not be an issue.