Move code depending on external_packages from Root to Namespace

[jjcnn]

Root::external_packages represents the bindings that exist in external libraries of the current package. As part of #6838 we want to move external_packages to Namespace so that we don't end up with references that the Rust compiler won't accept.

The first step in this process is to move all the code in Root that depends on external_packages to Namespace.