fix: template directories must be ignored by Go

`|` is not supported in filenames retrieved by the Go toolchain, as
Windows doesn't allow it in filenames. As we're zipping these files,
this isn't a concern for us, but it's still prevented by Go. To work
around this we create a dummy `go.mod` file which Go interprets as a
separate module and thus ignores.

Fixes #991

kotlin-runtime/external-module-template/go.mod

@@ -0,0 +1,3 @@
+module ignore
+
+go 1.22.0