otk: error when a otk.include inside a dict override the dict

We allow the following:
```yaml
otk.target.osbuild:
  sub:
    c: d
    otk.include: "foo.yaml"
```
The current semantic is that if the resolved value of `foo.yaml` is in
itself a dict the two are merged and if it's a non-dict value
the `sub` tree is replaced with the scalar value from `foo.yaml`.

However in the above example this can lead to unexpected results,
i.e. depending on how `foo.yaml` resolves `c: d` is removed or
kept.

This commit make it an error if an `otk.include` inside a non
empty dict returns a scalar. We *could* consider also making it
a warning but I fail to see a use-case right now (that is not
contrived) where this would not be unexpected/unwanted behavior.

src/otk/transform.py

@@ -94,6 +94,8 @@ def resolve_dict(ctx: Context, state: State, tree: dict[str, Any]) -> Any:
 
                 included = process_include(ctx, state, pathlib.Path(val))
                 if not isinstance(included, dict):
+                    if len(tree) > 0:
+                        raise ValueError(f"otk.include '{val}' overrides non-empty dict {tree} with '{included}'")
                     return included
 
                 tree.update(included)

test/data/error/08-include-scalar-siblings.err

@@ -0,0 +1 @@
+otk.include '08-include-scalar-siblings/foo.yaml' overrides non-empty dict {'c': 'd'} with 'foo'

test/data/error/08-include-scalar-siblings.yaml

@@ -0,0 +1,6 @@
+otk.version: 1
+
+otk.target.osbuild:
+  sub:
+    c: d
+    otk.include: "08-include-scalar-siblings/foo.yaml"

test/data/error/08-include-scalar-siblings/foo.yaml

@@ -0,0 +1 @@
+foo