Limit legacy state translations to custom components (#112295)

* Limit legacy state translations to custom components

We were trying to load **thousands** of `*.light.json`, `*.switch.json` files at run time that did not exist.

There have been replaced with entity translations: https://github.com/home-assistant/developers.home-assistant/pull/1557 https://github.com/home-assistant/core/pull/82701

https://github.com/home-assistant/core/pull/112023 will completely remove them, but
for now we will only load them for custom components to reduce the number
of files having to be examined

* reduce

* reduce

* reduce

* reduce

* comment

* coverage

* try to remove empty dict in loaded_translations fallback when missing

tests/testing_config/custom_components/test_legacy_state_translations/__init__.py

@@ -0,0 +1 @@
+"""Provide a mock package component."""

tests/testing_config/custom_components/test_legacy_state_translations/manifest.json

@@ -0,0 +1,7 @@
+{
+  "domain": "test_legacy_state_translations",
+  "name": "Test package for legacy state translations",
+  "documentation": "http://test-package.io",
+  "config_flow": true,
+  "version": "1.2.3"
+}

tests/testing_config/custom_components/test_legacy_state_translations_bad_data/__init__.py

@@ -0,0 +1 @@
+"""Provide a mock package component."""

tests/testing_config/custom_components/test_legacy_state_translations_bad_data/manifest.json

@@ -0,0 +1,7 @@
+{
+  "domain": "test_legacy_state_translations_bad_data",
+  "name": "Test package for legacy state translations",
+  "documentation": "http://test-package.io",
+  "config_flow": true,
+  "version": "1.2.3"
+}