Draft: [RFC] backend/session: make udev optional

libdemi is a lightweight and cross-platform alternative to libudev.

-Denum-backend=disabled requires mcf's[0] fork of libinput compiled with -Dtools=false -Dudev=false
-Denum-backend=demi requires my[1] fork of libinput compiled with -Denum-backend=demi

[0] https://github.com/oasislinux/libinput (master branch)
[1] https://github.com/illiliti/libinput (libdemi branch)

Marked as a draft since it is a POC.

Closes #2257