push: move bookmarks exchange in the exchange module
The bookmark exchange code was already extracted during a previous cycle. This
changesets moves the extracted function in this module. This function will read
and write data in the `pushoperation` object and It is preferable to have all
core function collaborating through this object in the same place.
This changeset is pure code movement only. Code change for direct consumption of
the `pushoperation` object will come later.
Testing that hghave does not crash when checking features
$ "$TESTDIR/hghave" --test-features 2>/dev/null