Add tests and code enhancement #201

Signed-off-by: Chin Yeung Li <tli@nexb.com>

Author: chinyeungli

src/fetchcode/download_urls.py

@@ -22,8 +22,9 @@
 from fetchcode.cran import CRAN
 from fetchcode.huggingface import Huggingface
 from fetchcode.pypi import Pypi
+from fetchcode.nix import Nix
 
-package_registry = [Pypi, CRAN, CPAN, Huggingface, Composer]
+package_registry = [Pypi, CRAN, CPAN, Huggingface, Composer, Nix]
 
 router = Router()