Fix infoflow type lookup for exclusion list.
Implement infoflow subgraph to handle min weight and excluded types.

Infoflow now will create a complete graph for the policy and then create
a subgraph to filter out nodes based on excluded types and edges based on
minimum weight.  The main graph will only need to be rebuilt if there is
a change in the permission map.  While this is a little more expensive for
seinfoflow, it should make interactive analysis in apol faster since
repeatedly deriving a subgraph will be faster than repeatedly rebuilding
the entire graph.
Change PermissionMap rule_weight to not raise exception.

We always want the rule_weight to work, even in the face of unmapped
permissions.  Other functions, e.g. for handling permission map editing
and saving may raise an exception.