Merge components in packages (#3555)

Co-authored-by: Paul Monigatti <pm@paul.pm>
Co-authored-by: Jesse Hills <3060199+jesserockz@users.noreply.github.com>

esphome/config_validation.py

@@ -13,6 +13,7 @@ import voluptuous as vol
 
 from esphome import core
 import esphome.codegen as cg
+from esphome.config_helpers import Extend
 from esphome.const import (
     ALLOWED_NAME_CHARS,
     CONF_AVAILABILITY,
@@ -490,6 +491,8 @@ def declare_id(type):
         if value is None:
             return core.ID(None, is_declaration=True, type=type)
 
+        if isinstance(value, Extend):
+            raise Invalid(f"Source for extension of ID '{value.value}' was not found.")
         return core.ID(validate_id_name(value), is_declaration=True, type=type)
 
     return validator