abuild: fix completions to vendor dir, warn on incorrect dir

Vendor completions (completions made/generated by packages) should be contained in /usr/share/fish/vendor_completions.d as per https://fishshell.com/docs/current/completions.html#where-to-put-completions:

• A directory for third-party software vendors to ship their own completions for their software, usually /usr/share/fish/vendor_completions.d
• The completions shipped with fish, usually installed in /usr/share/fish/completions;

[...]
If you are developing another program and would like to ship completions with your program, install them to the "vendor" completions directory. As this path may vary from system to system, the pkgconfig framework should be used to discover this path with the output of pkg-config --variable completionsdir fish.

Completions should not be installed into /usr/share/fish/completions as they will collide with completions shipped by fish (which already happened for few packages).

Signed-off-by: Jakub Panek me@panekj.dev