buildsys: avoid debcargo and dh-cargo on build

buildsys: avoid debcargo and dh-cargo on build

debcargo package requires all dependencies to be installed, even if
we basically just use it for d/control generation, which makes it a
bad choice for build dir generation here (i.e., with a native
package) as the DSC should be buildable without requiring all build
dependencies.

Add a target to update d/control manually (not much tested) and move
the build away from dh-cargo

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>