init: rewrite init process

Now use pure globbing to generate 100% valid function and
completion paths, effectively splitting the init process in two
steps, one which paths are added, and other when initialization
is done (sourcing init).

Package initialization code is done in a new function called
`initialize`, which takes care of the dependencies, ensuring
proper `init.fish` call order and handling compatibility.

This initialization code introduces a new interface for
`init.fish` hook, which deprecates the previously used event
model. The new interface injects three variables into `init.fish`:
path, package and bundle. This variables can be used by the
package to autoload paths, use bundled files, etc.

Also supports key bindings by sourcing
$OMF_CONFIG/key_bindings.fish and also key_bindings.fish in
packages (plugins and themes) root directories. This is done
when fish_user_key_bindings is called.