bug#39015: `isearch-complete1' should use `completion-at-point' or `completion-in-region'

[Alexander Shukaev]

Not sure why in the first place `isearch-complete1' uses some custom 
auto-completion implementation that is different from a conventional 
solution, perhaps historical reasons.  Thus, I believe that reusing 
either `completion-at-point' or `completion-in-region' to implement that 
functionality is much better in the long run.  This would also allow 
packages, which customize completion behavior, to hook into `isearch' 
auto-completion as well.  See also [1] for example.

[1] https://github.com/abo-abo/swiper/issues/1882