bundle2: move function building obsmarker-part in the bundle2 module We move it next to similar part building functions. We will need it for the "writenewbundle" logic. This will allow us to easily include obsmarkers in on-disk bundle, a necessary step before having `hg strip` also operate on markers. (Yes, the bundle2 module was already too large, but there any many interdependencies between its components so it is non-trivial to split, this is a quest for another adventure.)