[Bug] Setting outputFileName has no effect when set inside fileMetadata

**Describe the bug**
The file metadata supports `outputFileName` as a way to indicate the desired file name of a specific document. For example the following YAML metadata can be indicated at the top of a `README.md` file to make sure the result is emitted as `index.html`.

```yml
---
outputFileName: index.html
---
```

Since `outputFileName` is part of file metadata, one would expect it to be possible to specify it as part of the `fileMetadata` section in `docfx.json`, similar to other metadata entries.

Unfortunately, the below doesn't seem to work:

```json
"build": {
"fileMetadata": {
"_appTitle": {
"**/README.md": "index title"
},
"outputFileName": {
"**/README.md": "index.html"
}
}
}
```

In this case, `_appTitle` is applied successfully (only `README.md` files will have the custom title) but files will still be named `README.html`.

**Expected behavior**
All `README.md` files should be exported as `index.html` files.

**Context (please complete the following information):**
- OS: Windows
- Docfx version: 2.75.3