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/plugins/example/package.json

@@ -0,0 +1,13 @@
+{
+  "name": "flipper-plugin-example",
+  "version": "1.0.0",
+  "description": "An example for a Flipper plugin",
+  "main": "index.tsx",
+  "license": "MIT",
+  "keywords": ["flipper-plugin"],
+  "title": "Example Plugin",
+  "icon": "apps",
+  "bugs": {
+    "url": "https://fbflipper.com/"
+  }
+}