5955: Remove merge import code duplication r=jonas-schievink a=Veykril
This removes the code duplication caused by #5935, this also allows the assist to merge imports that have equal visibility and prevents merges of unequal visibility. This PR also fixes an iteration mistake in the mentioned PR:
Turns out I made a mistake when writing the `segment_iter` function, I was assuming that the `children` of a path will just be the segments, which is obviously not the case. This also brings insertion order of shorter paths in line with how `rustfmt` orders them.
Co-authored-by: Lukas Wirth <lukastw97@gmail.com>
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