Change return value of Schema::childNodes

I'll need to use this method to generate what can be parsed as paths in
the upcoming path parser rework. As a side effect, path completion
generation gets a little bit simpler, because now I don't have to parse
the strings of format "[module:]node" and just trust whatever the schema
gives me.

Change-Id: I881cdbbd8254b846c21cee1ac0a3b7af1e40a696
diff --git a/src/schema.hpp b/src/schema.hpp
index 4dd9a2a..052386e 100644
--- a/src/schema.hpp
+++ b/src/schema.hpp
@@ -70,5 +70,5 @@
     virtual std::optional<std::string> description(const std::string& location) const = 0;
     virtual yang::Status status(const std::string& location) const = 0;
 
-    virtual std::set<std::string> availableNodes(const boost::variant<dataPath_, schemaPath_, module_>& path, const Recursion recursion) const = 0;
+    virtual std::set<ModuleNodePair> availableNodes(const boost::variant<dataPath_, schemaPath_, module_>& path, const Recursion recursion) const = 0;
 };