Update the maven package finder

🍰 Context

The Maven package finder could return all existing packages under given conditions (no project and group set).

Even worse, it's returning all the packages from all types (eg. it's not using the .maven scope from the Packages::Package model). 🙀

See #284686 (closed)

What does this MR do?

• If a project and a group is not set, the finder will use Packages::Package.none as the base scope.
• Re organize the related specs
  • Using a subject
  • Using shared_examples

📸 Screenshots (strongly suggested)

n /a

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• [-] Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team