[SPARK-19715][STRUCTURED STREAMING] Option to Strip Paths in FileSource

## What changes were proposed in this pull request?

Today, we compare the whole path when deciding if a file is new in the FileSource for structured streaming. However, this would cause false negatives in the case where the path has changed in a cosmetic way (i.e. changing `s3n` to `s3a`).

This patch adds an option `fileNameOnly` that causes the new file check to be based only on the filename (but still store the whole path in the log).

## Usage

```scala
spark
  .readStream
  .option("fileNameOnly", true)
  .text("s3n://bucket/dir1/dir2")
  .writeStream
  ...
```
## How was this patch tested?

Added a test case

Author: Liwei Lin <lwlin7@gmail.com>

Closes #17120 from lw-lin/filename-only.