50_completion: Try to find all available completion functions

50_completion: Try to find all available completion functions

Try to add all folders possibly containing completion functions to the
fpath before calling compinit. See <https://github.com/ohmyzsh/ohmyzsh/issues/4614>
for a discussion of this topic, for example. It boils down to:

1. We have to call compinit early,
2. but plugins can add completions later, that won't be found ...