Implement maintaining object relationship graph

For each object maintain parent-child relationship graph, i.e. list of
parent objects ('parents' property) and list of children objects ('children'
property).

Objects without parent(s) are objects specified in init manifest.
Objects without children are object of types that do not reuse other types.

docs/src/cdist-cache.rst

@@ -61,6 +61,14 @@ Object cache overview
 ~~~~~~~~~~~~~~~~~~~~~
 Each object under :strong:`object` directory has its own structure.
 
+autorequire
+    file containing a list of object auto requirements
+
+children
+    file containing a list of object children, i.e. objects of types that this
+    type reuses (along with 'parents' it is used for maintaining parent-child
+    relationship graph)
+
 code-local
     code generated from gencode-local, present only if something is
     generated
@@ -80,6 +88,15 @@ parameter
     directory containing type parameter named files containing parameter
     values   
 
+parents
+    file containing a list of object parents, i.e. objects of types that reuse
+    this type (along with 'children' it is used for maintaining parent-child
+    relationship graph); objects without parents are objects specified in init
+    manifest
+
+require
+    file containing a list of object requirements
+
 source
     this type's source (init manifest)