Make sure that limited top-level exports are exposed from flipper-plugin

Summary: This prefixes APIs of `flipper-plugin`, that are used by Flipper, but should not be used by plugins directly, with `_`. Also added tests to make sure we are always intentional when extending the exposed APIs

Reviewed By: passy

Differential Revision: D24991700

fbshipit-source-id: ed3700efa188fca7f5a14d5c68250598cf011e42

desktop/app/src/utils/__tests__/exportData.node.tsx

@@ -24,7 +24,7 @@ import {State as PluginsState} from '../../reducers/plugins';
 import {renderMockFlipperWithPlugin} from '../../test-utils/createMockFlipperWithPlugin';
 import {
   TestUtils,
-  SandyPluginDefinition,
+  _SandyPluginDefinition,
   createState,
   PluginClient,
 } from 'flipper-plugin';
@@ -1052,7 +1052,7 @@ test('test determinePluginsToProcess to ignore archived clients', async () => {
   ]);
 });
 
-const sandyTestPlugin = new SandyPluginDefinition(
+const sandyTestPlugin = new _SandyPluginDefinition(
   TestUtils.createMockPluginDetails(),
   {
     plugin(