by checking if the current namespace decl is in our code, so we have to
scan less stuff, which results in a 10% perf improvement for me

Change-Id: Idf0e30d57b6d0dcd13daa9ed679c28b9d233d387
Reviewed-on: https://gerrit.libreoffice.org/58942
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>