[SPARK-19633][SS] FileSource read from FileSink

## What changes were proposed in this pull request?

Right now file source always uses `InMemoryFileIndex` to scan files from a given path.

But when reading the outputs from another streaming query, the file source should use `MetadataFileIndex` to list files from the sink log. This patch adds this support.

## `MetadataFileIndex` or `InMemoryFileIndex`
```scala
spark
  .readStream
  .format(...)
  .load("/some/path") // for a non-glob path:
                      //   - use `MetadataFileIndex` when `/some/path/_spark_meta` exists
                      //   - fall back to `InMemoryFileIndex` otherwise
```
```scala
spark
  .readStream
  .format(...)
  .load("/some/path/*/*") // for a glob path: always use `InMemoryFileIndex`
```

## How was this patch tested?

two newly added tests

Author: Liwei Lin <lwlin7@gmail.com>

Closes #16987 from lw-lin/source-read-from-sink.