Move path functions to their own file

ale_linters/d/dmd.vim

@@ -5,7 +5,7 @@ function! s:FindDUBConfig(buffer) abort
     " Find a DUB configuration file in ancestor paths.
     " The most DUB-specific names will be tried first.
     for l:possible_filename in ['dub.sdl', 'dub.json', 'package.json']
-        let l:dub_file = ale#util#FindNearestFile(a:buffer, l:possible_filename)
+        let l:dub_file = ale#path#FindNearestFile(a:buffer, l:possible_filename)
 
         if !empty(l:dub_file)
             return l:dub_file