This allows "vendors" (i.e. third-party upstreams interested in
supporting fish) to add auto-loaded functions and eager-loaded
configuration "snippets", while still allowing both the user and the administrator to
fully override all of that.
This has been inspired by systemd's configuration hierarchy, and implements a similar scheme
whereby files with the same name in higher-ranking directories override files in lower-ranking ones.
Fixes#1956
Assists other packages in finding the path to install completions: call
`pkg-config --variable=completionsdir fish` or so (like
bash-completion).
As discussed in #1485.
