Split produce_bundle into smaller methods

[teor2345]

To prepare for #3254, this PR splits DomainBundleProducer::produce_bundle() into smaller methods.

I've left the code in the same order as the original method, so it is easier to review. I'll do further cleanups in the test refactor PRs.

git diff --ignore-space-change might help with reviews, because some of the code is now at the top level of the new methods.

Code contributor checklist:

• I have read, understood and followed contributing guide

[nazar-pc]

About whitespace there is also a checkbox in GitHub's UI and you can add w=1 query parameter to URL in other places on GitHub where such button is not physically present on the page (like branch comparison page)

[NingLin-P]

LGTM, thanks!