This updates PerforceClient to add formal support for dependency

checks. PerforceClient.check_dependencies() now determines if p4 is

installed via the existing wrapper class's is_supported() method.
PerforceClient.get_local_path() now calls this instead of checking for

dependencies itself, emitting a warning if dependencies haven't already

been checked via setup() or has_dependencies().
Unit tests have been updated to include new dependency-related tests.

These tests aren't sensitive to p4 anyway as they already simulate

responses rather than calling into it, so there's no change in behavior

here.

Unit tests pass on Python 3.7-3.11.