[Core] Support already loaded packages in [p]load (#2116)

redbot/core/errors.py

@@ -0,0 +1,16 @@
+import importlib.machinery
+
+
+class RedError(Exception):
+    """Base error class for Red-related errors."""
+
+
+class PackageAlreadyLoaded(RedError):
+    """Raised when trying to load an already-loaded package."""
+
+    def __init__(self, spec: importlib.machinery.ModuleSpec, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self.spec: importlib.machinery.ModuleSpec = spec
+
+    def __str__(self) -> str:
+        return f"There is already a package named {self.spec.name.split('.')[-1]} loaded"