patch 8.2.4173: cannot use an import in 'foldexpr'

Problem:    Cannot use an import in 'foldexpr'.
Solution:   Set the script context to where 'foldexpr' was set. (closes #9584)
            Fix that the script context was not set for all buffers.

src/eval.c

@@ -772,7 +772,7 @@ call_func_retlist(
     return rettv.vval.v_list;
 }
 
-#ifdef FEAT_FOLDING
+#if defined(FEAT_FOLDING) || defined(PROTO)
 /*
  * Evaluate "arg", which is 'foldexpr'.
  * Note: caller must set "curwin" to match "arg".
@@ -780,14 +780,19 @@ call_func_retlist(
  * give error messages.
  */
     int
-eval_foldexpr(char_u *arg, int *cp)
+eval_foldexpr(win_T *wp, int *cp)
 {
+    char_u	*arg;
     typval_T	tv;
     varnumber_T	retval;
     char_u	*s;
+    sctx_T	saved_sctx = current_sctx;
     int		use_sandbox = was_set_insecurely((char_u *)"foldexpr",
 								   OPT_LOCAL);
 
+    arg = wp->w_p_fde;
+    current_sctx = wp->w_p_script_ctx[WV_FDE];
+
     ++emsg_off;
     if (use_sandbox)
 	++sandbox;
@@ -818,6 +823,7 @@ eval_foldexpr(char_u *arg, int *cp)
 	--sandbox;
     --textwinlock;
     clear_evalarg(&EVALARG_EVALUATE, NULL);
+    current_sctx = saved_sctx;
 
     return (int)retval;
 }