Detect google-cloud-location mixin and automatically add the dependency

[dazuma]

privateca/v1 recently added the location client as a mixin. We currently release the location client classes in a separate gem google-cloud-location, so we needed to add that gem as a custom dependency to privateca. We should investigate how to get the generator to detect that it's needed and add the dependency automatically. See, e.g. how GemPresenter#iam_dependency? does something similar for handling the dependency for IAM.

[dazuma]

This is handled in the new mixin code.