Prefix _all_ our completions with an underscore?

[scop]

Looking into #1385 I started to wonder if we should prefix all completion files we ship with an underscore, no matter if there's a known alternative completion or not.

If done this way, we would no longer need to be in reactive mode when new alternatives pop up.

This would incur the small penalty of a failing stat call for pretty much all our completion loads in most setups (for the underscoreless names), but I think that's negligible.

Thoughts?

[akinomyoga]

In February, I opened a discussion at #1329.

Thoughts?

I wrote my thoughts there. In particular, #1329 (comment) is specifically related to #1385, but it also involves other similar issues, which I think can be discussed together and solved consistently.