Define a separate version of s:is_local_plug for Windows

plug.vim

@@ -212,6 +212,10 @@ if s:is_win
   function! s:dirpath(path)
     return s:path(a:path) . '\'
   endfunction
+
+  function! s:is_local_plug(repo)
+    return a:repo =~? '^[a-z]:'
+  endfunction
 else
   function! s:rtp(spec)
     return s:dirpath(a:spec.dir . get(a:spec, 'rtp', ''))
@@ -224,6 +228,10 @@ else
   function! s:dirpath(path)
     return s:trim(a:path) . '/'
   endfunction
+
+  function! s:is_local_plug(repo)
+    return a:repo[0] =~ '[/$~]'
+  endfunction
 endif
 
 function! s:err(msg)
@@ -363,10 +371,6 @@ function! s:extract_name(repo)
   return substitute(fnamemodify(a:repo, ':t'), '\.git$', '', '')
 endfunction
 
-function! s:is_local_plug(repo)
-  return a:repo[0] =~ '[/$~]' || a:repo =~? '^[a-z]:'
-endfunction
-
 function! s:install(...)
   call s:update_impl(0, a:000)
 endfunction