PackageManager: Move class' extra searchPath/Package to an internal location
PackageManager: Move class' extra searchPath/Package to an internal location

Those fields have been aggregated in Location and them being in `PackageManager`
stands out as a double duty (the `PackageManager` is both a `Location`
and act as a `Location` manager). By using a `Location` for them we can
start to streamline the code.

Geod24/dub