Move runner services to their own namespace

What does this MR do and why?

Describe in detail what your merge request does and why.

Since we're extracting quite a few new services to handle logic that was previously spread out, it makes sense to create a runners directory to keep all runner-related services together.

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There are no changes to the logic itself in this MR, just file location changes. Unfortunately, Git doesn't consider the renamed files to be the same, so the diff is not as clear as it could be.

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