Move the base Context stuff to another module to keep context free for imports.

jedi/evaluate/context/__init__.py

@@ -1,5 +1,3 @@
-from jedi.evaluate.context.base import Context, iterate_contexts, \
-    TreeContext, ContextualizedName, ContextualizedNode, ContextSet, \
-    NO_CONTEXTS, iterator_to_context_set
 from jedi.evaluate.context.lazy import AbstractLazyContext, LazyKnownContext, \
     LazyKnownContexts, LazyTreeContext, LazyUnknownContext, get_merged_lazy_context
+from jedi.evaluate.context.module import ModuleContext