[substitutions] [packages] Address review feedback

- Seed `resolve_include` context with `command_line_substitutions` so
  `substitutions: !include ${var}.yaml` can reference CLI-provided vars
  in the include filename (parallels the `packages: !include` path).
- Validate shape of resolved substitutions in `do_packages_pass` and raise
  `cv.Invalid` under `CONF_SUBSTITUTIONS` instead of letting `UserDict()`
  fail with a low-level exception on a non-mapping.
- Fixture 17 exercises the CLI-templated include filename.

tests/unit_tests/fixtures/substitutions/17-substitutions_include_cli_var.approved.yaml

@@ -0,0 +1,6 @@
+substitutions:
+  subs_file: 15-substitutions_inc
+  wifi_password: sub_password
+wifi:
+  ssid: main_ssid
+  password: sub_password

tests/unit_tests/fixtures/substitutions/17-substitutions_include_cli_var.input.yaml

@@ -0,0 +1,8 @@
+command_line_substitutions:
+  subs_file: 15-substitutions_inc
+
+substitutions: !include ${subs_file}.yaml
+
+wifi:
+  ssid: main_ssid
+  password: $wifi_password