decouple downloading logic from external features

the `bc_free_downloader.py` module now only does the logic of getting
info from bandcamp and downloading from bandcamp. the task of checking
if an album is already downloaded is delegated to the __main__ file

free_bandcamp_downloader/bandcamp_http_adapter.py

@@ -1,4 +1,5 @@
 from requests.adapters import HTTPAdapter
+from urllib3.util import Retry
 from urllib3.util.ssl_ import create_urllib3_context