fix: correct deep-remove and deep-preserve regexes (#2572)

In this PR we fix the `deep-remove` and `deep-preserve` regexes in
`.OwlBot.yaml` files in the monorepo by creating a temporary
`.OwlBot.hermetic.yaml` with the corrected paths.

### Why?
Because the paths specified in `deep-remove-regex` and
`deep-preserve-regex` [start from the library
name](https://github.com/googleapis/google-cloud-java/blob/14f7146c98fdf03de2f113215e03347a21e83f9a/java-alloydb/.OwlBot.yaml#L17),
so if we run `copy-code` from inside a library, these regexes won't
match any files.

### What about deep-copy? 
We don't need to correct the library path because `copy-code` sends the
files from `googleapis-gen` (or our built temp folder in our case) to
the `owl-bot-staging` folder. These regexes don't deal with the
monorepo, they deal with the source repo, which for our case is the temp
folder we build after calling generate_library. This is why `deep-copy`
was always being "respected".

### Proof
After running `python generate_repo.py generate --generation-config-yaml
google-cloud-java/generation_config.yaml --repository-path
google-cloud-java --target-library-api-shortname merchantapi &> out`, we
get

![image](https://github.com/googleapis/sdk-platform-java/assets/22083784/087e0df1-baf3-4939-94e0-61027797d19e)

---------

Co-authored-by: Joe Wang <106995533+JoeWang1127@users.noreply.github.com>

library_generation/postprocess_library.sh

@@ -62,7 +62,22 @@ fi
 # we determine the location of the .OwlBot.yaml file by checking if the target
 # folder is a monorepo folder or not
 if [[ "${is_monorepo}" == "true" ]]; then
-  owlbot_yaml_relative_path=".OwlBot.yaml"
+  # the deep-remove-regex and deep-preserve-regex of the .OwlBot.yaml
+  # files in the monorepo libraries assume that `copy-code` is run
+  # from the root of the monorepo. However, we call `copy-code` from inside each
+  # library, so a path like `/java-asset/google-.*/src` will not have
+  # any effect. We solve this by creating a temporary owlbot yaml with
+  # the patched paths.
+  # For example, we convert 
+  # - "/java-asset/google-.*/src"
+  # to
+  # - "/google-.*/src"
+
+  library_name=$(basename "${postprocessing_target}")
+  cat "${postprocessing_target}/.OwlBot.yaml" \
+    | sed "s/- \"\/${library_name}/ - \"/" \
+    > "${postprocessing_target}/.OwlBot.hermetic.yaml"
+  owlbot_yaml_relative_path=".OwlBot.hermetic.yaml"
 else
   owlbot_yaml_relative_path=".github/.OwlBot.yaml"
 fi
@@ -109,6 +124,11 @@ docker run --rm \
   --source-repo="${preprocessed_libraries_binding}" \
   --config-file="${owlbot_yaml_relative_path}"
 
+# clean the custom owlbot yaml
+if [[ "${is_monorepo}" == "true" ]]; then
+  rm "${postprocessing_target}/.OwlBot.hermetic.yaml"
+fi
+
 # we clone the synthtool library and manually build it
 mkdir -p /tmp/synthtool
 pushd /tmp/synthtool