Expose common known third-party packages section

[tbekolay]

Both pylint and isort sometimes need us to be explicit about third-party packages. Normally this happens when a local first-party file is the same as the name of a third-party package. Since we do this in some projects, it makes life a tiny bit easier if there is a place to specify this in .nengobones.yml that will list those packages for both pylint and isort.