[Python] We should be able to depend on just the builtin dialect

[mikeurbach]

In 0102a61, we removed a dependency that no longer works. However, it would be nice to generate the Python bindings, but just for the builtin dialect. I tried a couple different CMake targets to try to add this back, but couldn't get it working. We should get to the bottom of this, so we can do things like isinstance(foo, builtin.ModuleOp).