emit more awesome error message when importing a symbol would overrid…

…e an existing lexical

Previously that died with "Multiple declarations of lexical $symbol".
Next step: don't die when using the same module twice (don't know if that's
going to be quite as easy...)

src/Perl6/Module/Loader.pm

@@ -125,11 +125,16 @@ method stub_lexical_imports($name, $block_ast) {
     my %imports := self.get_imports($name);
     unless pir::isnull__IP(%imports) {
         for %imports {
-            $block_ast[0].push(PAST::Var.new(
-                :name($_.key), :scope('lexical'), :isdecl(1),
-                :viviself(PAST::Op.new( :pirop('null P')) )
-            ));
-            $block_ast.symbol($_.key, :scope('lexical'));
+            if $block_ast.symbol($_.key) {
+                pir::die("Can't import symbol " ~ $_.key
+                    ~ " because it already exists in this lexical scope\n");
+            } else {
+                $block_ast[0].push(PAST::Var.new(
+                    :name($_.key), :scope('lexical'), :isdecl(1),
+                    :viviself(PAST::Op.new( :pirop('null P')) )
+                ));
+                $block_ast.symbol($_.key, :scope('lexical'));
+            }
         }
     }
 }