[SPARK-20255] Move listLeafFiles() to InMemoryFileIndex

## What changes were proposed in this pull request

Trying to get a grip on the `FileIndex` hierarchy, I was confused by the following inconsistency:

On the one hand, `PartitioningAwareFileIndex` defines `leafFiles` and `leafDirToChildrenFiles` as abstract, but on the other it fully implements `listLeafFiles` which does all the listing of files. However, the latter is only used by `InMemoryFileIndex`.

I'm hereby proposing to move this method (and all its dependencies) to the implementation class that actually uses it, and thus unclutter the `PartitioningAwareFileIndex` interface.

## How was this patch tested?

`./build/sbt sql/test`

Author: Adrian Ionescu <adrian@databricks.com>

Closes #17570 from adrian-ionescu/list-leaf-files.