Issue #3310929: Remove distro_core and distro_core_recommended fixtures in favor of mocking ComposerUtility::getInstalledPackages() in a unit test

package_manager/tests/fixtures/distro_core/vendor/composer/installed.json

-{
-    "packages": [
-        {
-            "name": "drupal/test-distribution",
-            "version": "1.0.0",
-            "require": {
-                "drupal/core": "*"
-            }
-        },
-        {
-            "name": "drupal/core",
-            "version": "9.8.0"
-        }
-    ]
-}