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_helpers.py

@@ -1,10 +1,27 @@
 import json
 import os
 
+from esphome.const import CONF_ID
 from esphome.core import CORE
 from esphome.helpers import read_file
 
 
+class Extend:
+    def __init__(self, value):
+        self.value = value
+
+    def __str__(self):
+        return f"!extend {self.value}"
+
+    def __eq__(self, b):
+        """
+        Check if two Extend objects contain the same ID.
+
+        Only used in unit tests.
+        """
+        return isinstance(b, Extend) and self.value == b.value
+
+
 def read_config_file(path: str) -> str:
     if CORE.vscode and (
         not CORE.ace or os.path.abspath(path) == os.path.abspath(CORE.config_path)
@@ -36,7 +53,25 @@ def merge_config(full_old, full_new):
         if isinstance(new, list):
             if not isinstance(old, list):
                 return new
-            return old + new
+            res = old.copy()
+            ids = {
+                v[CONF_ID]: i
+                for i, v in enumerate(res)
+                if CONF_ID in v and isinstance(v[CONF_ID], str)
+            }
+            for v in new:
+                if CONF_ID in v:
+                    new_id = v[CONF_ID]
+                    if isinstance(new_id, Extend):
+                        new_id = new_id.value
+                        if new_id in ids:
+                            v[CONF_ID] = new_id
+                            res[ids[new_id]] = merge(res[ids[new_id]], v)
+                            continue
+                    else:
+                        ids[new_id] = len(res)
+                res.append(v)
+            return res
         if new is None:
             return old