Fix issue with __defaults__ not propagating over gaps in the tree of BUILD files.

[kaos]

Fix bug introduced in #15836

As reported by @ced-f on Slack:

I have a top level django/BUILD file which contains:

__defaults__({
    (python_tests): dict(dependencies=["django/django_core/settings.py"])
})

And a BUILD file under django/sub/tests/classes/BUILD which just defines:

python_tests(
    name="tests",
)

The dependencies goal in the subdirectory does not reference django/django_core/settings.py
If I extend the contents of django/sub/tests/classes/BUILD the dependency django/django_core/settings.py gets shown correctly:

__defaults__({
    (python_tests): dict(dependencies=["django/django_core/settings.py"])
})
python_tests(
    name="tests",
)

[stuhood]

Mm, good call. And more parallel too.