Properly delete files when a package is removed

What does this MR do?

Both CarrierWave and ProjectStatistics relies on record deletion to cleanup storage and update statistics.

We delete Packages instead of PackageFiles and the record deletion is performed by the DB.

This MR introduces a dependent: :destroy on Package-PackageFiles relation to make sure that when a package is deleted all its files are properly disposed of.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Performance 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

Relevant issues:

• related to gitlab-org/gitlab-ee#12583
• related to gitlab-org/gitlab-ee#12581