[ClassLoader] made DebugClassLoader idempotent

The DebugClassLoader will wrap itself if `enable()` is called multiple time, such as when running functional tests.

Please merge to 2.2 and master ASAP.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~