Conditionally inject dependencies of functions

For some functions, it is possible that the functions it depends
upon are not explicitly present, e.g. for the following aggregate
function:

CREATE AGGREGATE array_agg_mult(anyarray) (
    SFUNC = array_cat,
    STYPE = anyarray,
    INITCOND = '{}'
);

We got the following error when trying to dbtoyaml this database

KeyError: (u'public', 'array_cat', u'anyarray, anyarray'), but
the dependency is upon pg_catalog.array_cat, so should not even be
made explicit.

pyrseas/dbobject/function.py

@@ -660,7 +660,8 @@ def get_implied_deps(self, db):
             args = self.arguments.replace(' ORDER BY', ',')
         else:
             args = self.stype + ', ' + self.arguments
-        deps.add(db.functions[sch, fnc, args])
+        if (sch, fnc, args) in db.functions:
+            deps.add(db.functions[sch, fnc, args])
         for fn in ('finalfunc', 'mfinalfunc'):
             if getattr(self, fn) is not None:
                 func = getattr(self, fn)