Swap plugin cache to pickle-able values when done

doc/whatsnew/fragments/7635.bugfix

@@ -0,0 +1,7 @@
+Fixes a multi-processing crash that prevents using any more than 1 thread on MacOS.
+
+The returned module objects and errors that were cached by the linter plugin loader
+cannot be reliably pickled. This means that ``dill`` would throw an error when
+attempting to serialise the linter object for multi-processing use.
+
+Closes #7635.

pylint/lint/pylinter.py

@@ -296,7 +296,7 @@ def __init__(
             str, list[checkers.BaseChecker]
         ] = collections.defaultdict(list)
         """Dictionary of registered and initialized checkers."""
-        self._dynamic_plugins: dict[str, ModuleType | ModuleNotFoundError] = {}
+        self._dynamic_plugins: dict[str, ModuleType | ModuleNotFoundError | bool] = {}
         """Set of loaded plugin names."""
 
         # Attributes related to registering messages and their handling
@@ -402,7 +402,17 @@ def load_plugin_configuration(self) -> None:
                     "bad-plugin-value", args=(modname, module_or_error), line=0
                 )
             elif hasattr(module_or_error, "load_configuration"):
-                module_or_error.load_configuration(self)
+                # Mypy is not smart enough to realize that we only call
+                # this line if the object has the attr we are looking at.
+                # Hence the one-line type: ignore[union-attr] here.
+                module_or_error.load_configuration(self)  # type: ignore[union-attr]
+        # We re-set all the dictionary values to True here to make sure the dict
+        # is pickle-able. This is only a problem in multiprocessing/parallel mode.
+        # (e.g. invoking pylint -j 2)
+        self._dynamic_plugins = {
+            modname: not isinstance(val, ModuleNotFoundError)
+            for modname, val in self._dynamic_plugins.items()
+        }
 
     def _load_reporters(self, reporter_names: str) -> None:
         """Load the reporters if they are available on _reporters."""