Infer alias when only one interpretation
likewise for the alias matching version of that same basic code change for https://github.com/coleifer/peewee/issues/965

not that this is now valid:

``` python
User.alias().select().where(User.username=='charlie')
```

but this is still not:

``` python
User.alias().select().join(User.alias(), on=User.username=='charlie')
```
