Entries in configuration without a consuming dependency cause errors

Having a root entry in the configuration which is not referred to by any other root entry as a dependency causes an error in dependency resolution:

```
A = { my_b: "#p/ref B" }
B = { ... }
C = { ... }
```

will raise an exception like 
```
pyntegrant/map.py, line 78 in <module>:
node in g:
g was <networkx.classes.digraph.DiGraph object at 0x7f929b078940>
node was 'C'
```