Move plugins to "sonar/desktop/plugins"

Summary:
Plugins moved from "sonar/desktop/src/plugins" to "sonar/desktop/plugins".

Fixed all the paths after moving.

New "desktop" folder structure:
- `src` - Flipper desktop app JS code executing in Electron Renderer (Chrome) process.
- `static` - Flipper desktop app JS code executing in Electron Main (Node.js) process.
- `plugins` - Flipper desktop JS plugins.
- `pkg` - Flipper packaging lib and CLI tool.
- `doctor` - Flipper diagnostics lib and CLI tool.
- `scripts` - Build scripts for Flipper desktop app.
- `headless` - Headless version of Flipper desktop app.
- `headless-tests` - Integration tests running agains Flipper headless version.

Reviewed By: mweststrate

Differential Revision: D20344186

fbshipit-source-id: d020da970b2ea1e001f9061a8782bfeb54e31ba0

desktop/scripts/start-dev-server.ts

@@ -182,12 +182,7 @@ async function addWebsocket(server: http.Server) {
         io.emit('refresh');
       },
       {
-        excludes: [
-          '**/__tests__/**/*',
-          '**/node_modules/**/*',
-          '**/.*',
-          'plugins/**/*', // plugin changes are tracked separately, so exlcuding them here to avoid double reloading.
-        ],
+        excludes: ['**/__tests__/**/*', '**/node_modules/**/*', '**/.*'],
       },
     );
   } catch (err) {