fix leaking product info when building cache
    
    When building the product cache, the information of which products a
    package/component is part of could be leaked from one item to the next
    because it's only every reset once per call.
    
    This was introduced when we moved the functionality of the ProductCache
    class into BugzillaProxy.build_product_cache().
    
    Signed-off-by: Nils Philippsen <nils@redhat.com>