[SPARK-15895][SQL] Filters out metadata files while doing partition discovery

## What changes were proposed in this pull request?

Take the following directory layout as an example:

```
dir/
+- p0=0/
   |-_metadata
   +- p1=0/
      |-part-00001.parquet
      |-part-00002.parquet
      |-...
```

The `_metadata` file under `p0=0` shouldn't fail partition discovery.

This PR filters output all metadata files whose names start with `_` while doing partition discovery.

## How was this patch tested?

New unit test added in `ParquetPartitionDiscoverySuite`.

Author: Cheng Lian <lian@databricks.com>

Closes #13623 from liancheng/spark-15895-partition-disco-no-metafiles.