Add a base-class for a resolver, and start raising exceptions if problems are encountered

Add a base-class for a resolver, and start raising exceptions if problems are encountered (rather than logging them).

Includes:

• Creation of an ABC for a Resolver
• Ability to isolate the cmmnbuild_dep_manager.Manager for testing, and added some tests
• Resolution of !10 (comment 2911676)

cmmnbuild_dep_manager/resolver/__init__.py

+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]
+
+
+class Resolver(ABC):
+    """
+    An abstract base class for a dependency resolver.
+
+    Known implementations include
+    :class:`cmmnbuild_dep_manager.resolver.cbng_web.CbngWebResolver`
+    and
+    :class:`cmmnbuild_dep_manager.resolver.gradle.GradleResolver`
+
+    """
+    #: The name of the variable that must be available on a module which wishes
+    #: to be able to have its dependencies resolved by this resolver (str).
+    dependency_variable = '__the_name_of_the_module_attribute_for_deps__'
+
+    #: A short description of the resolver, for logging/info purposes
+    #: only (str).
+    description = 'The description of the resolver.'
+
+    @classmethod
+    @abstractmethod
+    def is_available(cls):
+        pass
+
+    @abstractmethod
+    def __init__(self, dependencies):
+        pass
+
+    @abstractmethod
+    def save_jars(self, dir):
+        pass
+
+    @classmethod
+    @abstractmethod
+    def get_help(cls, classnames, class_info):
+        pass