Garbage collect old custodian policies based on prefix.

    We attempt to introspect to find the event sources for a policy
    but without the old configuration this is implicit.

def resources_gc_prefix(options, policy_config, policy_collection):
    """Garbage collect old custodian policies based on prefix.

    We attempt to introspect to find the event sources for a policy
    but without the old configuration this is implicit.
    """

    # Classify policies by region
    policy_regions = {}
    for p in policy_collection:
        if p.execution_mode == 'poll':
            continue
        policy_regions.setdefault(p.options.region, []).append(p)

    regions = get_gc_regions(options.regions)
    for r in regions:
        region_gc(options, r, policy_config, policy_regions.get(r, []))