Fix config validation not erroring on global options in wrong scope (#10950)
Fix config validation not erroring on global options in wrong scope (#10950)

This used to not complain:

```toml
[source]
backend_packages = ["pants.backend.python"]
```

Closes https://github.com/pantsbuild/pants/issues/3713.

[ci skip-rust]
[ci skip-build-wheels]
benjyw/pants,benjyw/pants,benjyw/pants,pantsbuild/pants,benjyw/pants,jsirois/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,pantsbuild/pants,jsirois/pants,jsirois/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants