dbghelp: Cleanups for module loading (lookups on module name and image name are two different things)

- split up module_find_by_name in two different functions:
  + reused module_find_by_name which looks upon the module name
  + added module_is_already_loaded which looks upon the image name
- cleanup module loading using these functions (removed extra parameter
  to pe_load_module_from_pcs)