The case for symlinked directories being duplicated a lot isn't there,
but there *is* a usecase for adding the symlink rather than the
target, and that's homebrew.
E.g. homebrew installs ruby into /usr/local/Cellar/ruby/2.7.1_2/bin,
and links to it from /usr/local/opt/ruby/bin. If we add the target, we
would miss updates.
Having path entries that point to the same location isn't a big
problem - it's a path lookup, so it takes a teensy bit longer. The
canonicalization is mainly so paths don't end up duplicated via weird
spelling and so relative paths can be used.
This is a function you can either execute once, interactively, or
stick in config.fish, and it will do the right thing.
Some options are included to choose some slightly different behavior,
like setting $PATH directly instead of $fish_user_paths, or moving
already existing components to the front/back instead of ignoring
them, or appending new components instead of prepending them.
The defaults were chosen because they are the most safe, and
especially because they allow it to be idempotent - running it again
and again and again won't change anything, it won't even run the
actual `set` because it skips that if all components are already in.
Fixes#6960.
