"function --argument" is not a thing, it's "--argument-names". This only
accidentally works because our getopt is awful and allows abbreviated
long options.
Similarly, one argparse test used "--d" instead of "-d" or "--def".
__fish_complete_suffix accepts a first argument containing a
brace-expansion, like
__fish_complete_suffix '.{c,cpp,py}'
We're gonna be removing the `eval` that does that shortly, so let's
remove all uses in our code.
