Fix paths issue with transitive excludes.

[kaos]

Error for test before the fix was:

E       pants.engine.internals.scheduler.ExecutionError: 1 Exception encountered:
E
E       Engine traceback:
E         in select
E           ..
E         in pants.backend.project_info.paths.paths
E           `paths` goal
E
E       Traceback (most recent call last):
E         File "/private/var/folders/y2/6t5bbk554wv5h6w_qlgpcpnm0000gr/T/pants-sandbox-gbcg46/src/python/pants/engine/internals/selectors.py", line 623, in native_engine_generator_send
E           res = rule.send(arg) if err is None else rule.throw(throw or err)
E         File "/private/var/folders/y2/6t5bbk554wv5h6w_qlgpcpnm0000gr/T/pants-sandbox-gbcg46/src/python/pants/backend/project_info/paths.py", line 133, in paths
E           for path in find_paths_breadth_first(adjacency_lists, root.address, destination.address):
E         File "/private/var/folders/y2/6t5bbk554wv5h6w_qlgpcpnm0000gr/T/pants-sandbox-gbcg46/src/python/pants/backend/project_info/paths.py", line 73, in find_paths_breadth_first
E           for dep in adjacency_lists[target]:
E       KeyError: Address(3rdparty#lib-pluginA)