Using Inline to [re]define default fields causes "maximum recursion depth exceeded"

### Problem

Another semi-silly one. Users can probably guess that this is a bad idea, but I tried it anyway.

This happens after telling import to [R]emove old:

```sh
[many more lines like the following]
File "beets\beetsplug\inline.py", line 102, in _dict_for
out = dict(obj)
File "beets\beets\dbcore\db.py", line 237, in __getitem__
return getters[key](self)
File "beets\beetsplug\inline.py", line 120, in _func_func
func.__globals__.update(_dict_for(obj))
File "beets\beetsplug\inline.py", line 102, in _dict_for
out = dict(obj)
File "beets\beets\dbcore\db.py", line 237, in __getitem__
return getters[key](self)
File "beets\beetsplug\inline.py", line 120, in _func_func
func.__globals__.update(_dict_for(obj))
File "beets\beetsplug\inline.py", line 102, in _dict_for
out = dict(obj)
File "beets\beets\dbcore\db.py", line 281, in keys
base_keys = list(self._fields) + list(self._values_flex.keys())
RecursionError: maximum recursion depth exceeded while calling a Python object
```

Led to this problem:

```
album_fields:
album: |
return 'asdf'
```

### Setup

* OS: Win10 x64 Powershell
* Python version: 3.6.0
* beets version: 1.4.4+ (git intermediate before 1.4.5)
* Turning off plugins made problem go away (yes/no): probably

When I have beets like this, it lists two "album" fields in `beet fields`.

### Potential fix

Maybe quit beets with an error message about using the same fields as beets defaults?