Organize packages API code using namespaces

What does this MR do?

Organize packages presenters and finders used in the different packages custom API.

Creates a dedicated namespace (which translates to a folder) for each package type.

The reason motivating this change is that, as we add other package types support, these future files will end up in a dedicated folder.

This MR does the following:

• organizes class per package type in ee/app/finders, ee/app/presenters and ee/app/services. It's mainly creating a folder/namespace per package type and move classes there. Classes themselves were not modified (except adding the module and updating the name).
• adds missing specs for Packages::Maven::FindOrCreatePackageService.

Screenshots

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

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