[sdk-platform-java] Hermetic Build does not support `.OwlBot-hermetic.yaml` for _new_ standalone libraries

_standalone == non-monorepo_

Inside an empty folder with a single `generation_config.yaml`, the generated library fails to be fully "unpacked" (part of postprocessing) due to the default `.OwlBot-hermetic.yaml` assuming a monorepo structure:

https://github.com/googleapis/sdk-platform-java/blob/49a7ae50071e75fe0d161a4eb9360a4fe4e6147b/hermetic_build/library_generation/templates/owlbot.yaml.monorepo.j2#L17

Where `module_name` is the monorepo folder (e.g. `java-alloydb`).

This results in the wrong folder structure:

```
hi on] diegomarquezp:test-new-library$ ls
total 44K
drwxr-x--- 5 diegomarquezp primarygroup 4.0K May 27 20:54 .
drwxr-x--- 75 diegomarquezp primarygroup 4.0K May 23 19:17 ..
-rw-r----- 1 diegomarquezp primarygroup 402 May 23 19:37 generation_config.yaml
drwxr-xr-x 2 diegomarquezp primarygroup 4.0K May 27 19:30 .github
drwx------ 7 diegomarquezp primarygroup 4.0K May 27 19:30 output
-rw-r--r-- 1 diegomarquezp primarygroup 1.1K May 27 19:30 owlbot.py
-rw-r--r-- 1 diegomarquezp primarygroup 8.8K May 27 19:30 README.md
-rw-r--r-- 1 diegomarquezp primarygroup 699 May 27 19:30 .repo-metadata.json
drwxr-xr-x 6 diegomarquezp primarygroup 4.0K May 27 19:30 v1alpha
-rw-r--r-- 1 diegomarquezp primarygroup 0 May 27 19:30 versions.txt
```

Notice the `v1alpha` folder. This folder is meant to be "unpacked" by the postprocessing workflow.

The expected output is similar to what we see in HW repos.

cc: @zhumin8