Add variable substitutions for !include (#3510)

tests/unit_tests/fixtures/yaml_util/includes/included.yaml

@@ -0,0 +1,2 @@
+---
+ssid: ${name}

tests/unit_tests/fixtures/yaml_util/includes/list.yaml

@@ -0,0 +1,2 @@
+---
+- ${var1}

tests/unit_tests/fixtures/yaml_util/includes/scalar.yaml

@@ -0,0 +1 @@
+${var1}

tests/unit_tests/fixtures/yaml_util/includetest.yaml

@@ -0,0 +1,17 @@
+---
+substitutions:
+  name: original
+
+wifi: !include
+  file: includes/included.yaml
+  vars:
+    name: my_custom_ssid
+
+esphome:
+  # should be substituted as 'original', not overwritten by vars in the !include above
+  name: ${name}
+  name_add_mac_suffix: true
+  platform: esp8266
+  board: !include { file: includes/scalar.yaml, vars: { var1: nodemcu } }
+
+  libraries: !include { file: includes/list.yaml, vars: { var1: Wire } }

tests/unit_tests/test_yaml_util.py

@@ -0,0 +1,13 @@
+from esphome import yaml_util
+from esphome.components import substitutions
+
+
+def test_include_with_vars(fixture_path):
+    yaml_file = fixture_path / "yaml_util" / "includetest.yaml"
+
+    actual = yaml_util.load_yaml(yaml_file)
+    substitutions.do_substitution_pass(actual, None)
+    assert actual["esphome"]["name"] == "original"
+    assert actual["esphome"]["libraries"][0] == "Wire"
+    assert actual["esphome"]["board"] == "nodemcu"
+    assert actual["wifi"]["ssid"] == "my_custom_ssid"