## What changes were proposed in this pull request?
In https://github.com/apache/spark/pull/23383, the file source V2 framework is implemented. In the PR, `FileIndex` is created as a member of `FileTable`, so that we can implement partition pruning like 0f9fcabb4a in the future(As data source V2 catalog is under development, partition pruning is removed from the PR)
However, after write path of file source V2 is implemented, I find that a simple write will create an unnecessary `FileIndex`, which is required by `FileTable`. This is a sort of regression. And we can see there is a warning message when writing to ORC files
```
WARN InMemoryFileIndex: The directory file:/tmp/foo was not found. Was it deleted very recently?
```
This PR is to make `FileIndex` as a lazy value in `FileTable`, so that we can avoid creating unnecessary `FileIndex` in the write path.
## How was this patch tested?
Existing unit test
Closes#23774 from gengliangwang/moveFileIndexInV2.
Authored-by: Gengliang Wang <gengliang.wang@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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