Change datasets directory layout

Make it so that there are no symlinks needed.

category_classification/datasets/en/download_common.py

@@ -1 +0,0 @@
-../download_common.py

category_classification/datasets/{en/prepare_dataset.ipynb → prepare_en_dataset.ipynb}

@@ -17,7 +17,7 @@
    "source": [
     "from pathlib import Path\n",
     "\n",
-    "from download_common import *"
+    "from datasets_common import *"
    ]
   },
   {