Namespace the finders for gitlab_com for usage billing

The following discussion from !14552 should be addressed:

@vburton started a discussion: (+1 comment)
Suggestion (can be done in a follow-up MR): It would be good to add this and related finders that now only have namespace methods to a GitLab.com specific module now (ex: like GitlabCom). This would match the conventions we have for other classes in CustomersDot today where we need to distinguish between SM and .com

ex:
```
module Billing
  module Usage
    module GitlabCom
      class SubscriptionsFinder
```