[wip] getModuleGroup removal using single CL for all modules

[SylvainJuge]

Alternative to #18707 by using a single shared classloader for all instrumentation modules.

Hypotheses to verify

• we should not get any shared class issues as they are loaded exactly once in the same CL (tests should pass as they currently do)
• we loose the isolation between instrumentation modules
  • for internal instrumentation this is probably not an issue as there is no overlap in classes.
  • for extensions, they are currently isolated by default, so we need to evaluate how likely it could cause issues.