import-checker: refactor sys.path prefix check (issue4129) This patch refactors the logic in contrib/import-checker.py responsible for checking the beginnings of the paths in sys.path. In particular, it adds a variable that defines the set of allowed prefixes. The primary purpose of this change is to make it easier to add more allowed prefixes. This will be useful in resolving issue4129, which involves making the function list_stdlib_modules() work when run from a virtualenv.