[packages] Add more information and deprecation deadline for "single package" includes (#12280)

esphome/components/packages/__init__.py

@@ -91,7 +91,16 @@ def validate_source_shorthand(value):
 
 def deprecate_single_package(config):
     _LOGGER.warning(
-        "Including a single package under `packages:` is deprecated. Use a list instead."
+        """
+        Including a single package under `packages:`, i.e., `packages: !include mypackage.yaml` is deprecated.
+        This method for including packages will go away in 2026.7.0
+        Please use a list instead:
+
+        packages:
+          - !include mypackage.yaml
+
+        See https://github.com/esphome/esphome/pull/12116
+        """
     )
     return config
 

tests/component_tests/packages/test_packages.py

@@ -176,10 +176,7 @@ def test_single_package(
 
     assert actual == expected
 
-    assert (
-        "Including a single package under `packages:` is deprecated. Use a list instead."
-        in caplog.text
-    )
+    assert "This method for including packages will go away in 2026.7.0" in caplog.text
 
 
 def test_package_append(basic_wifi, basic_esphome):