refactor: modularize protobuf module loader and add directory safety checks

[bhagathkrishnacdac]

Description
This PR refactors _import_modules in the pybess module importer to resolve cognitive complexity, eliminate nested functions, and enforce better directory safety constraints.

Key Changes

• Modularized Discovery Logic: Decomposed the nested dynamic importer logic into independent, single-responsibility functions:
  • _discover_protobuf_modules: Scans search paths for available protobuf definitions.
  • _get_modules_from_directory: Processes individual directory structures to find valid _msg_pb2 targets.
  • _keep_protobuf_name: Standardized filter to isolate CLI-bound argument, config, and response schemas.
  • _handle_collision_error: Formats and prints compilation namespace collisions before exit.
• Directory Exist Check: Added os.path.isdir(dirpath) validation to _get_modules_from_directory to prevent unhandled OSError crashes if subdirectory structures (such as ports/) are absent in plugin folders.