Revisit directory-to-bundle logic

[weyfonk]

While looking at this code and that one in the context of reading directories for determining which ones to create bundles from, it seems that we could probably simplify some of this logic:

• [ ] this seems to browse each subdirectory multiple times, once from the filepath.Walk loop and once from shouldCreateBundleFromThisPath logic. Can we do better?
• [ ] consider using WalkDir, supposedly more efficient than Walk.

Related to this comment.

[manno]

Let's wait how much of this is necessary after https://github.com/rancher/fleet/issues/210

[weyfonk]

Already done.