Refactor how strip_root argument works for Codebase

A proposed change in scancode.io is that the root Resource of a project codebase will be named `.` and all other Resources of that codebase will not have the root path prefix in its path. We should change how the `strip_root` argument for `Codebase()` works where the root resource is renamed to `.` and the path set to `.`, and the root path prefixes are removed from the remaining Resource paths.