Add functions and configuration snippets hierarchy

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

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