Add a proof-of-concept direct interface to CBNG.

Following on from the outage of cbng-web this morning, I wondered what it would take to just call CBNG directly. This change is complimented by the proof-of-concept implementation at https://gitlab.cern.ch/pelson/cbng-py, but without it everything should still work as before (or not if cbng-web is down).

cmmnbuild_dep_manager/resolver/__init__.py

@@ -3,8 +3,8 @@ from abc import ABC, abstractmethod
 
 def resolvers():
     """Return all supported resolvers, ordered by precedence"""
-    from . import gradle, cbng_web
-    return [cbng_web.CbngWebResolver, gradle.GradleResolver]
+    from . import gradle, cbng_web, cbng_py
+    return [cbng_py.CbngPyResolver, cbng_web.CbngWebResolver, gradle.GradleResolver]
 
 
 class Resolver(ABC):