chore(librarian): allow onboarding new packages

[parthea]

This PR resolves the following error by ignoring libraries which which do not have entries in release-please-manifest.json when onboarding new packages. These are likely libraries that have already onboarded. We will also preserve existing entries in state.yaml for these libraries.

(py392) partheniou@partheniou-vm-3:~/git/google-cloud-python/scripts/configure_state_yaml$ python3 configure_state_yaml.py 
Traceback (most recent call last):
  File "/usr/local/google/home/partheniou/git/google-cloud-python/scripts/configure_state_yaml/configure_state_yaml.py", line 94, in <module>
    configure_state_yaml()
  File "/usr/local/google/home/partheniou/git/google-cloud-python/scripts/configure_state_yaml/configure_state_yaml.py", line 67, in configure_state_yaml
    "version": release_please_manifest[f"packages/{package_name}"],
KeyError: 'packages/google-cloud-dlp'

Towards googleapis/librarian#761