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/fresco/__tests__/__snapshots__/index.node.tsx.snap

@@ -0,0 +1,32 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`notifications for leaks 1`] = `
+<React.Fragment>
+  <Styled(div)>
+    CloseableReference leaked for
+     
+    <Text
+      code={true}
+    >
+      com.facebook.imagepipeline.memory.NativeMemoryChunk
+    </Text>
+    (identity hashcode: 
+    deadbeef
+    ).
+  </Styled(div)>
+  <Styled(div)>
+    <Text
+      bold={true}
+    >
+      Stacktrace:
+    </Text>
+  </Styled(div)>
+  <Styled(div)>
+    <Text
+      code={true}
+    >
+      &lt;unavailable&gt;
+    </Text>
+  </Styled(div)>
+</React.Fragment>
+`;