substitute included vars with abs path

file a:
```
foo={x:1}{include b}
bar={x:2}
```
file b:
```
y: ${bar.x}
```
this causes `{unresolved,['foo.bar.x']}` exception.
according to the spec:
> Substitutions in included files are looked up at two different paths; first, relative to the root of the included file; second, relative to the root of the including configuration.