Add local package.

Samples/UIWidgetsSamples_2019_4/Packages/manifest.json

     "com.unity.collab-proxy": "1.2.16",
     "com.unity.ide.rider": "1.1.4",
     "com.unity.ide.vscode": "1.2.1",
+    "com.unity.uiwidgets": "file:../../../com.unity.uiwidgets",
     "com.unity.test-framework": "1.1.14",
     "com.unity.textmeshpro": "2.0.1",
     "com.unity.timeline": "1.2.15",