[SPARK-18103][SQL] Rename *FileCatalog to *FileIndex

## What changes were proposed in this pull request?

To reduce the number of components in SQL named *Catalog, rename *FileCatalog to *FileIndex. A FileIndex is responsible for returning the list of partitions / files to scan given a filtering expression.

```
TableFileCatalog => CatalogFileIndex
FileCatalog => FileIndex
ListingFileCatalog => InMemoryFileIndex
MetadataLogFileCatalog => MetadataLogFileIndex
PrunedTableFileCatalog => PrunedInMemoryFileIndex
```

cc yhuai marmbrus

## How was this patch tested?

N/A

Author: Eric Liang <ekl@databricks.com>
Author: Eric Liang <ekhliang@gmail.com>

Closes #15634 from ericl/rename-file-provider.