patch 8.2.4330: Vim9: no error if script imports itself

Problem:    Vim9: no error if script imports itself.
Solution:   Give an error when a script imports itself.

src/errors.h

@@ -2802,7 +2802,8 @@ EXTERN char e_function_reference_invalid[]
 	INIT(= N_("E1086: Function reference invalid"));
 EXTERN char e_cannot_use_index_when_declaring_variable[]
 	INIT(= N_("E1087: Cannot use an index when declaring a variable"));
-// E1088 unused
+EXTERN char e_script_cannot_import_itself[]
+	INIT(= N_("E1088: Script cannot import itself"));
 EXTERN char e_unknown_variable_str[]
 	INIT(= N_("E1089: Unknown variable: %s"));
 EXTERN char e_cannot_assign_to_argument[]

src/testdir/test_vim9_import.vim

@@ -501,6 +501,15 @@ def Test_import_fails()
 
   delete('Xthat.vim')
 
+  lines =<< trim END
+      vim9script
+      export var item = 'hello'
+      import './Xyourself.vim'
+  END
+  writefile(lines, 'Xyourself.vim')
+  assert_fails('source Xyourself.vim', 'E1088:')
+  delete('Xyourself.vim')
+
   mkdir('Ximport')
 
   writefile(['vim9script'], 'Ximport/.vim')

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4330,
 /**/
     4329,
 /**/

src/vim9script.c

@@ -516,6 +516,12 @@ handle_import(
 	goto erret;
     }
 
+    if (sid == current_sctx.sc_sid)
+    {
+	emsg(_(e_script_cannot_import_itself));
+	goto erret;
+    }
+
     import_gap = gap != NULL ? gap : &SCRIPT_ITEM(import_sid)->sn_imports;
     for (i = 0; i < import_gap->ga_len; ++i)
     {