Do not require explicit dependency list when building in a side tag

[lachmanfrantisek]

From the discussion and meeting with the Fedora KDE SIG, for wider adoption, it might be required to get the dependency information from elsewhere. (Despite it's possible to script the config, it's not maintainable to have it defined in multiple places.)

(For them the source of truth is in the specfile.)

A few ideas regarding the implementation:

• Provide an action/config option to load this info from a script.
  • Easy to implement but requires sandbox which is not needed for the current Koji/Bodhi jobs.
• Allow people to configure the source.
  • The implementation is inside Packit.
  • No sandbox needed.
• External global dependency config.
  • Hard to maintain.

[lachmanfrantisek]

We discussed this today at the arch meeting and had resource concerns about running this on each dist-git/Koji event.

I've asked the SIG about how often this changes so we better know where we can generate this and if we can cache this.

Also, a broader question is if we shouldn't pick a different approach for such ecosystems and not work on a single-package-for+Fedora-branch level in general. (Run the whole group update somehow together, similarly to the scripts these groups tend to have, usually without PRs..;-)