dpkg-gencontrol: Rework Installed-Size field default value computation

Switch from «du» to File::Find, and accumulate size usage per filesystem
object, on 1 KiB units. Use the actual size only for regular files and
symlinks, and just 1 KiB for any other filesystem object type.

This guarantees a constant and reproducible size regardless of the
build system filesystem being used.

Document how the value is computed, and that it is just a size
approximation.

Closes: #650077