[CogManager, Utils] Handle missing cogs correctly, add some helpful algorithms (#1989)

* Handle missing cogs correctly, add some helpful algorithms

For cog loading, only show "cog not found" if the module in question was the one
that failed to import. ImportErrors within cogs will show an error as they should.

- deduplicator, benchmarked to be the fastest
- bounded gather and bounded async as_completed
- tests for all additions

* Requested changes + wrap as_completed instead

So I went source diving and realized as_completed works the way I want it to,
and I don't need to reinvent the wheel for cancelling tasks that remain
if the generator is `break`ed out of. So there's that.

redbot/core/core_commands.py

@@ -77,9 +77,17 @@ class CoreLogic:
         for name in cog_names:
             try:
                 spec = await bot.cog_mgr.find_cog(name)
-                cogspecs.append((spec, name))
-            except RuntimeError:
-                notfound_packages.append(name)
+                if spec:
+                    cogspecs.append((spec, name))
+                else:
+                    notfound_packages.append(name)
+            except Exception as e:
+                log.exception("Package import failed", exc_info=e)
+
+                exception_log = "Exception during import of cog\n"
+                exception_log += "".join(traceback.format_exception(type(e), e, e.__traceback__))
+                bot._last_exception = exception_log
+                failed_packages.append(name)
 
         for spec, name in cogspecs:
             try:
@@ -95,6 +103,7 @@ class CoreLogic:
             else:
                 await bot.add_loaded_package(name)
                 loaded_packages.append(name)
+
         return loaded_packages, failed_packages, notfound_packages
 
     def _cleanup_and_refresh_modules(self, module_name: str):
@@ -511,7 +520,7 @@ class Core(CoreLogic):
             loaded, failed, not_found = await self._load(cog_names)
 
         if loaded:
-            fmt = "Loaded {packs}"
+            fmt = "Loaded {packs}."
             formed = self._get_package_strings(loaded, fmt)
             await ctx.send(formed)