GoImpl: support relative (vendor) imports

  * attempt to use `-dir` argument from josharian/impl#18 if `impl`
    fails.
  * backwards compatible with previous `impl` versions.

autoload/go/impl.vim

@@ -35,8 +35,14 @@ function! go#impl#Impl(...) abort
 
   let result = go#util#System(printf("%s '%s' '%s'", binpath, recv, iface))
   if go#util#ShellError() != 0
-    call go#util#EchoError(result)
-    return
+    call go#util#EchoWarning("impl failed.  trying with package path")
+
+    let s:dirname = fnameescape(expand('%:p:h'))
+    let result = go#util#System(printf("%s -dir '%s' '%s' '%s'", binpath, s:dirname, recv, iface))
+    if go#util#ShellError() != 0
+      call go#util#EchoError(result)
+      return
+    end
   endif
 
   if result ==# ''