Use a dynamic segment not depending on the package id for Debian files

What does this MR do?

Debian has the notion of incoming, where just-uploaded files are stored until the .changes file is processed. The .changes file contains relevant metadata to move the files to the matching source package.

This MR ensure the dynamic segment doesn't depend on the package.

Item 4.3 of #5835 (comment 414103286), cherry-picked from !44746 (closed).

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