Refactor: various: Include new xml.h instead of msg_xml.h or xml_names.h
msg_xml.h has been replaced by xml_names.h.
xml.h includes xml_names.h. xml_names.h can be considered a helper
header file, and we'll include only xml.h.
Also, previously msg_xml.h and xml.h included each other circularly.
Many files included only msg_xml.h and not xml.h; it seems they were
relying on the transitive include. Here we standardize it to xml.h.
Signed-off-by: Reid Wahl <nrwahl@protonmail.com>