libinput: split utilities to a bin output

libinput builds some executables which are useful for debugging or
unusual configurations, but not necessary in normal use of the library.
(See libinput(1) and the other referenced man pages.)

Some of these programs require Python, which makes the closure
significantly larger. Splitting /libexec/ and /bin/ to a separate "bin"
output allows systems that don't need these utilities to avoid the
substantial size hit.