fix: check for explicit preset during onboarding config build

[LukasAuerbeck]

Changes

Currently renovate ignores whether the extends field is explicitly set in the onboardingConfig template. Instead the repo's are iterated, and if a preset repo is found overwritten. This becomes a problem when the user:

• Explicitly sets a preset version (e.g. like in the added test case)
• There are mulitple preset configs within an organization's group (e.g. the default preset is org/renovate-config, the repo is in org/group/repo and there also exists a org/group/renovate-config that is only required in a handful of manually selected repos)

Context

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[github-actions]

Please create a GitHub Discussion before continuing with this PR.

Thank you for your PR, but we need to discuss the requirements and implementation first.

The maintainers believe that there is a lack of - or misalignment of - requirements about this PR. We need to discuss the requirements and implementation first so that you don't write code that won't be merged.

This PR will be closed for now to avoid confusion, but you can reopen it after the discussion has been resolved.

Thanks, the Renovate team