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'