patch 8.2.0368: Vim9: import that redefines local variable does not fail

Problem:    Vim9: import that redefines local variable does not fail.
Solution:   Check for already defined symbols.
diff --git a/src/vim9script.c b/src/vim9script.c
index 6cea96b..ce5cfec 100644
--- a/src/vim9script.c
+++ b/src/vim9script.c
@@ -143,7 +143,8 @@
 	emsg(_(e_needs_vim9));
     else
     {
-	char_u *cmd_end = handle_import(eap->arg, NULL, current_sctx.sc_sid);
+	char_u *cmd_end = handle_import(eap->arg, NULL,
+						    current_sctx.sc_sid, NULL);
 
 	if (cmd_end != NULL)
 	    eap->nextcmd = check_nextcmd(cmd_end);
@@ -238,7 +239,7 @@
  * Returns a pointer to after the command or NULL in case of failure
  */
     char_u *
-handle_import(char_u *arg_start, garray_T *gap, int import_sid)
+handle_import(char_u *arg_start, garray_T *gap, int import_sid, void *cctx)
 {
     char_u	*arg = arg_start;
     char_u	*cmd_end;
@@ -278,6 +279,8 @@
 		    ++arg;
 	    as_len = (int)(arg - as_ptr);
 	    arg = skipwhite(arg);
+	    if (check_defined(as_ptr, as_len, cctx) == FAIL)
+		return NULL;
 	}
 	else if (*arg_start == '*')
 	{
@@ -389,6 +392,9 @@
 	    if (idx < 0 && ufunc == NULL)
 		return NULL;
 
+	    if (check_defined(name, name_len, cctx) == FAIL)
+		return NULL;
+
 	    imported = new_imported(gap != NULL ? gap
 				       : &SCRIPT_ITEM(import_sid)->sn_imports);
 	    if (imported == NULL)