authz/perforce: refactor `p4 protects` parsing

Created by: bobheadxi

No behaviour changes here. Pulls the p4 protects parsing that exists now into:

• a scanner that handles basic parsing and filtering of p4 protects output, and feeds structured data to a provided callback
• two utility functions that can be provided to the scanner to do the same thing they do today (parse repo inclusion/exclusions, and parse users)

Both of the above now live in protects.go

This will improve the ergonomics for us to do more p4 protects-related work, namely https://github.com/sourcegraph/sourcegraph/issues/26644