this lets us *dis*able filesystem explicitly, as is required by e.g. the
zfs-less installer images. currently that specifically is only easily
possible by adding an overlay that stubs out `zfs`, with the obvious
side-effect of also removing tooling that could run without the kernel
module loaded.
This includes disabling some features in the initrd by default, this is
only done when the new initrd is used. Namely, ext and bcache are
disabled by default. bcache gets an own enable option while ext is
detected like any other filesystem.
Using pkgs.lib on the spine of module evaluation is problematic
because the pkgs argument depends on the result of module
evaluation. To prevent an infinite recursion, pkgs and some of the
modules are evaluated twice, which is inefficient. Using ‘with lib’
prevents this problem.
