Improve vim's 'gf' for BUILD.gn files

tools/gn/misc/vim/autoload/gn.vim

 " Copyright 2017 The Chromium Authors. All rights reserved.
 " Use of this source code is governed by a BSD-style license that can be
 " found in the LICENSE file.
 
 function! gn#TranslateToBuildFile(name) abort
   " Strip '//' prefix
   let l:new_path = substitute(a:name, '\v^//', '', '')
 
   " Strip the build target name (necessary if 'isfname' contains ':')
   let l:new_path = substitute(l:new_path, '\v:.*$', '', '')
 
   " Append 'BUILD.gn', only if this is a directory and not a file
-  if isdirectory(l:new_path)
-    let l:new_path = substitute(l:new_path, '\v/?$', '/BUILD.gn', '')
+  " Prefer using maktaba if it's available, but fallback to an alternative
+  if exists('*maktaba#path#Basename')
+    " Check if the last part of the path appears to be a file
+    if maktaba#path#Basename(l:new_path) !~# '\V.'
+      let l:new_path = maktaba#path#Join([l:new_path, 'BUILD.gn'])
+    endif
+  else
+    " This will break if 'autochdir' is enabled
+    if isdirectory(l:new_path)
+      let l:new_path = substitute(l:new_path, '\v/?$', '/BUILD.gn', '')
+    endif
   endif
   return l:new_path
 endfunction