Add Packages::Debian::GenerateDistributionWorker

What does this MR do?

Item 3.1.5 of #5835 (comment 414103286).

This MR is getting really big. To sum up why:

• this MR was initially only implementing the generate distribution worker (and plugging it in ProcessChangesService)
• as the specs were similar with the service, I refactored those
• also, the packages were processed in the specs, I've moved this to factories instead
• there was a spelling error in a fixture, so the fixture was fixed and rebuilt
• this changed some checksums which were copied all over the specs. I updated them, and refactored some of those usages to take the checksums from the factories
• the worker was initially not idempotent, I had to rewrite the service to fix this
• some component files scopes were added/changed

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?). Not needed.
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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