Python Binding Unit Tests Rely on URDFs in Examples

[liangfok]

The unit tests in drake-distro/drake/bindings/python/pydrake/test/all access URDFs in drake-distro/drake/examples/. This violates the order-of-includes rule we're trying to enforce.

Related:

• Dependency visibility should be enforced #1987 - "Dependency visibility should be enforced"
• Add drake/util/scan_subdirectory_order.py #2533 - "Add drake/util/scan_subdirectory_order.py"
• Create drake/models/ for holding URDF and SDF files that are shared across examples and systems #2548 - "Shared models directory"

[jwnimmer-tri]

So between (1) a revised understanding of what's allowed to depend on what (tests may in general depend on anything anywhere), and (2) its pydrake (which TRI doesn't curate), I think that this is no longer an issue. Resolved-invalid.