Eliminate Package class

[ikretz]

This PR proposes to eliminate the Package class, which superclasses PackageScanner and ProjectScanner.

Reasons to do this:

1. There is no context in which a PackageScanner and a ProjectScanner can be used interchangeably without a great deal of difficulty. These classes have only one common method, scan_local(), that expect different kinds (as opposed to types) of arguments: a path to a local package directory or archive for PackageScanner versus a path to a requirements.txt file for ProjectScanner

2. It seems the only purpose of Package is to give scanner.get_scanner() a definite, non-union return type. However, we end up paying elsewhere for this simplicity, namely via the explicit cast in cli._scan() needed only to satisfy the type checker, so it's not clear we gain anything from it. This PR splits get_scanner() into two functions to eliminate the cast

[christophetd]

Thanks! LGTM, this is probably legacy from relatively old code.