[commands] Don't raise ExtensionNotFound for ImportErrors in modules

Now loading an extension that _contains_ a failed import will fail
with ExtensionFailed, rather than ExtensionNotFound.

discord/ext/commands/bot.py

@@ -27,7 +27,7 @@ DEALINGS IN THE SOFTWARE.
 import asyncio
 import collections
 import inspect
-import importlib
+import importlib.util
 import sys
 import traceback
 import re
@@ -588,12 +588,17 @@ class BotBase(GroupMixin):
                 if _is_submodule(name, module):
                     del sys.modules[module]
 
-    def _load_from_module_spec(self, lib, key):
+    def _load_from_module_spec(self, spec, key):
         # precondition: key not in self.__extensions
+        lib = importlib.util.module_from_spec(spec)
+        try:
+            spec.loader.exec_module(lib)
+        except Exception as e:
+            raise errors.ExtensionFailed(key, e) from e
+
         try:
             setup = getattr(lib, 'setup')
         except AttributeError:
-            del sys.modules[key]
             raise errors.NoEntryPointError(key)
 
         try:
@@ -603,7 +608,7 @@ class BotBase(GroupMixin):
             self._call_module_finalizers(lib, key)
             raise errors.ExtensionFailed(key, e) from e
         else:
-            self.__extensions[key] = lib
+            sys.modules[key] = self.__extensions[key] = lib
 
     def load_extension(self, name):
         """Loads an extension.
@@ -637,12 +642,11 @@ class BotBase(GroupMixin):
         if name in self.__extensions:
             raise errors.ExtensionAlreadyLoaded(name)
 
-        try:
-            lib = importlib.import_module(name)
-        except ImportError as e:
-            raise errors.ExtensionNotFound(name, e) from e
-        else:
-            self._load_from_module_spec(lib, name)
+        spec = importlib.util.find_spec(name)
+        if spec is None:
+            raise errors.ExtensionNotFound(name)
+
+        self._load_from_module_spec(spec, name)
 
     def unload_extension(self, name):
         """Unloads an extension.