Bug: Fix loading behavior with FlagExternalCorePlugins (#78388)

pkg/plugins/manager/testdata/plugin-with-dist/plugin/dist/MANIFEST.txt

@@ -0,0 +1,28 @@
+
+-----BEGIN PGP SIGNED MESSAGE-----
+Hash: SHA512
+
+{
+  "manifestVersion": "2.0.0",
+  "signatureType": "grafana",
+  "signedByOrg": "grafana",
+  "signedByOrgName": "Grafana Labs",
+  "plugin": "test-datasource",
+  "version": "1.0.0",
+  "time": 1661171059101,
+  "keyId": "7e4d0c6a708866e7",
+  "files": {
+    "plugin.json": "203ef4a613c5693c437a665cd67f95e2756a0f71b336b2ffb265db7c180d0b19"
+  }
+}
+-----BEGIN PGP SIGNATURE-----
+Version: OpenPGP.js v4.10.10
+Comment: https://openpgpjs.org
+
+wrgEARMKAAYFAmMDdXMAIQkQfk0ManCIZucWIQTzOyW2kQdOhGNlcPN+TQxq
+cIhm54zLAgdfVimeut6Gw9MrIACBZUSH0ht9p9j+iG6MDjpmEFIpqVJrem6f
+8wBv0/kmYU3LV9MWyPuUeRfBdccjQKSjEXlfEAIJAVmut9LcSKIykhWuQA+7
+VMVvJPXzlPkeoYsGYvzAlxh8i2UomCU15UChe62Gzq5V5HgGYkX5layIb5XX
+y2Pio0lc
+=/TR0
+-----END PGP SIGNATURE-----

pkg/plugins/manager/testdata/plugin-with-dist/plugin/dist/plugin.json

@@ -0,0 +1,16 @@
+{
+  "type": "datasource",
+  "name": "Test",
+  "id": "test-datasource",
+  "backend": true,
+  "executable": "test",
+  "state": "alpha",
+  "info": {
+    "version": "1.0.0",
+    "description": "Test",
+    "author": {
+      "name": "Will Browne",
+      "url": "https://willbrowne.com"
+    }
+  }
+}

pkg/plugins/manager/testdata/plugin-with-dist/plugin/plugin.json

@@ -0,0 +1,16 @@
+{
+  "type": "datasource",
+  "name": "Test",
+  "id": "test-datasource",
+  "backend": true,
+  "executable": "test",
+  "state": "alpha",
+  "info": {
+    "version": "2.0.0",
+    "description": "Test",
+    "author": {
+      "name": "Will Browne",
+      "url": "https://willbrowne.com"
+    }
+  }
+}