feat(python): warn about inefficient apply json.loads if json is local import

[MarcoGorelli]

The following doesn't raise on main:

def func():
    import json

    df = pl.DataFrame({'a': '{"a":[1,2,3]}'}).select(pl.col('a').apply(lambda x: json.loads(x)))


func()

[alexander-beedie]

Nicely done; very clean :)