[substitutions] [packages] Fix substitutions: !include file.yaml regression

Resolve a deferred IncludeFile before validating the substitutions shape in
do_substitution_pass, and before wrapping it in UserDict in do_packages_pass.

Fixes esphome/esphome#15848

tests/unit_tests/fixtures/substitutions/15-substitutions_as_include.approved.yaml

@@ -0,0 +1,5 @@
+substitutions:
+  wifi_password: sub_password
+wifi:
+  ssid: main_ssid
+  password: sub_password

tests/unit_tests/fixtures/substitutions/15-substitutions_as_include.input.yaml

@@ -0,0 +1,5 @@
+substitutions: !include 15-substitutions_inc.yaml
+
+wifi:
+  ssid: main_ssid
+  password: $wifi_password

tests/unit_tests/fixtures/substitutions/15-substitutions_inc.yaml

@@ -0,0 +1 @@
+wifi_password: sub_password

tests/unit_tests/fixtures/substitutions/16-substitutions_as_include_with_packages.approved.yaml

@@ -0,0 +1,5 @@
+substitutions:
+  wifi_password: sub_password
+wifi:
+  ssid: main_ssid
+  password: sub_password

tests/unit_tests/fixtures/substitutions/16-substitutions_as_include_with_packages.input.yaml

@@ -0,0 +1,9 @@
+substitutions: !include 15-substitutions_inc.yaml
+
+packages:
+  wifi_pkg:
+    wifi:
+      password: $wifi_password
+
+wifi:
+  ssid: main_ssid