ENH/BF: no heavy import of downloaders at the top level of interfaces…

…/download_url.py

Makes it slower for --help and might result in warnings etc even when --help

datalad/interface/download_url.py

@@ -24,7 +24,6 @@
 from ..dochelpers import exc_str
 from ..support.param import Parameter
 from ..support.constraints import EnsureStr, EnsureNone
-from ..downloaders import Providers
 
 from logging import getLogger
 lgr = getLogger('datalad.api.download-url')
@@ -71,6 +70,9 @@ def __call__(self, urls, path=None, overwrite=False, stop_on_failure=False):
         list of str
           downloaded successfully files
         """
+
+        from ..downloaders import Providers
+
         urls = assure_list_from_str(urls)
 
         if len(urls) > 1: