Reorganize files

This moves a bunch of files out of libvirt-daemon and libvirt-clients and into packages that make more sense for them.

Two new binary packages are introduced: libvirt-login-shell, for the existing virt-login-shell command, and libvirt-daemon-driver-storage-iscsi-direct, for a storage driver that we hadn't enabled in Debian until now.

I've taken great care when moving files between packages, and verified that upgrading from both 6.8.0-1 (bullseye) and 5.0.0-4 (buster) works as expected, with no errors reported and no user intervention required.

I'm aiming squarely at experimental with these changes: my plan is to get !76 (merged) (which this MR applies on top of) merged, upload the safe and boring 6.9.0 rebase to unstable, and let these more aggressive changes simmer in experimental for a bit. I have additional changes in the same vein that I'm working on as well.