Bug: no-extraneous-packages packageDir should not override readPkgUp logic

[hulkish]

Currently, this logic is flawed so that if you set packageDir option to either string or array - it will override the readPkgUp logic.

I think this logic should remain, here is my use case:

1. Mono repo with X packages
2. I am prohibiting use of devDependencies for all nested packages.
3. Instead, all devDependencies are defined in the workspace/repo root.
4. All circular dependencies on sibling mono repo packages are defined in peerDependencies.
5. This means, since it's only a peer dep - using packageDir cause this rule to miss the defined peer dep because it will only search what is defined in packageDir.