style(_comp_compgen): call _comp_compgen_filedir directly

[akinomyoga]

I think it's easy enough to remember that if no _comp_compgen features are needed, calling _comp_compgen_FOO directly is ok (and preferred) for efficiency. So I guess that's what I'd prefer, but whatever we go with would be good to be documented somewhere, e.g. in the style guide, or the related function commentary.

Originally posted by @scop in #954 (comment)

The change _comp_compgen filedir => _comp_compgen_filedir is another large one and makes the GitHub interface unuseful, so I decided to separate the PR for it.

Originally posted by @akinomyoga in #954 (comment)

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c91e6c4 is the rewrite.

I'm including the variable name changes in _comp_compgen_filedir to avoid conflicts that can be introduced when _comp_comgen -v var filedir is used (08477b4).

but whatever we go with would be good to be documented somewhere, e.g. in the style guide, or the related function commentary.

I added a @remarks in the function code document (ea3a88e) but haven't added it to the style guide. It seems even usage of xfunc is currently undocumented in doc/*.md, so it is consistent to describe both _comp_xfunc and _comp_compgen if we are to add commentary of the _comp_compgen usage in the style guide.

[scop]

Thanks!