Topology viewer seems broken

When running the topology viewer, I get an error:
```
$ ./scion.sh topodot -s topology/default.topo
Traceback (most recent call last):
  File "/tmp/Bazel.runfiles_wvrs2tvk/runfiles/_main/tools/topodot.py", line 26, in <module>
    from topology.topo import LinkEP, LinkType, TopoID
ModuleNotFoundError: No module named 'topology'
```