[SPARK-18760][SQL] Consistent format specification for FileFormats

## What changes were proposed in this pull request?
This patch fixes the format specification in explain for file sources (Parquet and Text formats are the only two that are different from the rest):

Before:
```
scala> spark.read.text("test.text").explain()
== Physical Plan ==
*FileScan text [value#15] Batched: false, Format: org.apache.spark.sql.execution.datasources.text.TextFileFormatxyz, Location: InMemoryFileIndex[file:/scratch/rxin/spark/test.text], PartitionFilters: [], PushedFilters: [], ReadSchema: struct<value:string>
```

After:
```
scala> spark.read.text("test.text").explain()
== Physical Plan ==
*FileScan text [value#15] Batched: false, Format: Text, Location: InMemoryFileIndex[file:/scratch/rxin/spark/test.text], PartitionFilters: [], PushedFilters: [], ReadSchema: struct<value:string>
```

Also closes #14680.

## How was this patch tested?
Verified in spark-shell.

Author: Reynold Xin <rxin@databricks.com>

Closes #16187 from rxin/SPARK-18760.