libsane-hpaio: move 56-hpmud.rules to /usr

libsane-hpaio contains udev rules which are installed to /lib; these files need to be moved to /usr/lib as part of Debian's usr-merge effort. Because libsane-hpaio is Multi-Arch: same, an unfortunate corner-case can occur whereby shared files (such as the udev rules) may be erroneously removed on upgrades (please see DEP17[1] P7: Shared multiarch file loss).

Move udev rules to /usr, including mitigation for the file loss scenario.