Rename utils.move_symlink to utils.update_symlink

Non-functional changes:

• Renamed utils.move_symlink to utils.update_symlink to better reflect
  what it does.
• Renamed many of its variables so I could understand it better.
• Added notes about how the existing implementation is not atomic, but
  could be made to be so.

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Related MRs:

• !351 (merged)
• !350 (merged)
• !352 (merged)
• !348 (merged) (This MR)
• master