Fix plugin source maps in dev mode

Summary:
Start bundling source maps together with the source code itself in the dev mode. Therefore it is no longer required to add a link to the external source map file in dev mode.
In prod mode we still ship them separately.

Reviewed By: mweststrate

Differential Revision: D39775064

fbshipit-source-id: 6c56df7a3fce084c07a8618a63dbd8ae4741348c

desktop/pkg-lib/src/runBuild.tsx

@@ -66,7 +66,7 @@ async function runBuild({
       // It is an optional dependency for rollup that we use in react-devtools
       'fsevents',
     ],
-    sourcemap: 'external',
+    sourcemap: dev ? 'inline' : 'external',
     minify: !dev,
     plugins: intern ? [resolveFbStubsToFbPlugin] : undefined,
   });