Set up separate package build

Summary:
Added a microbundle based build setup to the data-source folder to be able to package just that folder.

For simplicity / iteration speed, this is only used to publish externally. Our own code still references the source files directly.

More strict separation can be done later if there is external adoption.

Reviewed By: nikoant

Differential Revision: D28056699

fbshipit-source-id: a011b615cfffeff8ecb879bd7281a71085cea965

desktop/flipper-plugin/src/ui/data-table/DataTable.tsx

@@ -28,7 +28,7 @@ import {
   DataSourceRendererStatic,
   DataSource,
   DataSourceVirtualizer,
-} from '../../data-source';
+} from '../../data-source/index';
 import {
   computeDataTableFilter,
   createDataTableManager,