[Core] Make Requires.verify() wait until rules are loaded (#2857)

* Make Requires.verify() wait until rules are loaded

Also ensures `Requires` objects are reset when unloaded, particularly in case a `Command` object manages to stay in memory between cog unload and load, and its permissions rules change between those events.

Also, this PR re-ordered some of the event loop policy stuff, because it was required that the event loop policy be set before creating any `Requires` objects. This may or may not have an effect on other `get_event_loop()` calls elsewhere (either in our code, a dependency's, or asyncio's). Either way, these effects would be a *correction*, and any bugs that arise from it are likely to have been occurring silently beforehand.

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Remove calls to `remove_listener()` in permissions

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Fix adding rules for permissions cog/commands itself

Also addresses feedback

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Clean up indentation when setting uvloop policy

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

* Use `set(walk_commands())` to traverse `Group` subcommands

Signed-off-by: Toby Harradine <tobyharradine@gmail.com>

redbot/__main__.py

@@ -10,6 +10,21 @@ import sys
 
 import discord
 
+# Set the event loop policies here so any subsequent `get_event_loop()`
+# calls, in particular those as a result of the following imports,
+# return the correct loop object.
+if sys.platform == "win32":
+    asyncio.set_event_loop_policy(asyncio.WindowsProactorEventLoopPolicy())
+elif sys.implementation.name == "cpython":
+    # Let's not force this dependency, uvloop is much faster on cpython
+    try:
+        import uvloop
+    except ImportError:
+        uvloop = None
+        pass
+    else:
+        asyncio.set_event_loop_policy(uvloop.EventLoopPolicy())
+
 import redbot.logging
 from redbot.core.bot import Red, ExitCodes
 from redbot.core.cog_manager import CogManagerUI
@@ -21,18 +36,6 @@ from redbot.core.dev_commands import Dev
 from redbot.core import __version__, modlog, bank, data_manager
 from signal import SIGTERM
 
-# Let's not force this dependency, uvloop is much faster on cpython
-if sys.implementation.name == "cpython":
-    try:
-        import uvloop
-    except ImportError:
-        uvloop = None
-        pass
-    else:
-        asyncio.set_event_loop_policy(uvloop.EventLoopPolicy())
-
-if sys.platform == "win32":
-    asyncio.set_event_loop(asyncio.ProactorEventLoop())
 
 log = logging.getLogger("red.main")