pimport.zipimporter related cached data
    that might somehow later get used when attempting to load data from the new
    distribution and thus cause such load operations to fail. Note that when
    tracking down such remaining stale data, we can not catch every conceivable
    usage from here, and we clear only those that we know of and have found to
    cause problems if left alive. Any remaining caches should be updated by
    whomever is in charge of maintaining them, i.e. they should be ready to
    handle us replacing their zip archives with new distributions at runtime.

    N)