Collision detection doesn't compare symlinks to files or directories

[Kyle Fazzari]

Snapcraft runs a collision detection algorithm when attempting to merge two parts. This algorithm has two related issues:

1. It only compares files, which means files (e.g. symlinks) that collide with directories are missed until the migration is attempted, at which point cryptic "destination is a directory" errors are raised.
2. If a file in both parts is a symlink, it immediately assumes they don't collide, without checking where they're pointing. If they point to different places they _should_ collide.

[Kyle Fazzari (kyrofa)]

Fix here: https://github.com/snapcore/snapcraft/pull/1064

[Leo Arias (elopio)]

Back to triaged because the PR never landed.

[Kyle Fazzari (kyrofa)]

Re-proposed same PR.