patch 8.2.2774: Vim9: cannot import an existing name even when using "as"

Problem:    Vim9: cannot import an existing name even when using "as".
Solution:   Do not check for an existing name when using "as". (closes #8113)

src/testdir/test_vim9_script.vim

@@ -1280,6 +1280,8 @@ def Test_import_as()
 
   var import_lines =<< trim END
     vim9script
+    var one = 'notused'
+    var yes = 777
     import one as thatOne from './XexportAs'
     assert_equal(1, thatOne)
     import yes as yesYes from './XexportAs'

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2774,
 /**/
     2773,
 /**/

src/vim9script.c

@@ -600,7 +600,8 @@ handle_import(
 	    }
 	    else
 	    {
-		if (check_defined(name, len, cctx, FALSE) == FAIL)
+		if (as_name == NULL
+			      && check_defined(name, len, cctx, FALSE) == FAIL)
 		    goto erret;
 
 		imported = new_imported(gap != NULL ? gap