Be less strict about "All conditional function variants must have identical signatures"

This is one of the followups on #649. Example code:

```
try:
    from zlip import decompress
except ImportError:
    def decompress(val):
        raise RuntimeError
```

This produces "error: All conditional function variants must have identical signatures" on the `def` line. It would be nice if this were more lenient (e.g. computing the intersection of the imported definition and the given def).
