[SPARK-3011][SQL] _temporary directory should be filtered out by sqlContext.parquetFile

fix compile error on hadoop 0.23 for the pull request #1924. 

`listStatus(path)` call can be expensive, especially when the `path` is an S3 URI. In that case, an HTTP request is issued to get all the `FileStatus` objects. and it may take hundred of milliseconds  to return (depending on how may HDFS files there are). That's why I `children` is used.

And yea, I also find definition of "Parquet file" somewhat confusing, and even the official Parquet documentations doesn't provide a precise definition. IMO a `part-*` file is roughly equivalent to a row group of a Parquet file, but it also carries its own metadata and is thus self describing.