As part of the GNOME update to 2.18, I added a python flavour to libxml2. This seems to confuse pkg_add and triggers a bug (one more) in the dependency handling code. Incidentally, the code in question is written in Perl by Marc Espie.
To completely fix the bug requires removing pkg(1) completely, which I will not be able to do before MirOS #10. As a first step, the verbose output of pkg is now more meaningful, allowing (hopefully) to debug the issue.
The way dependencies are handled is deeply flawed and is one of the biggest weaknesses in the current MirPorts framework. pkg(1) converts the dependencies into static @pkgdep lines of the form "@pkgdep libxml-2.6.14-2". If a libspec cannot be resolved, the adding almost certainly fails *sigh*.