kmodtool: depmod path

Determine the location of depmod on the system, either /sbin/depmod or
/usr/sbin/depmod. Then use that path when generating the specfile.

Additionally, update the Requires lines to reference the package which
provides depmod rather than the binary itself. For CentOS/RHEL 7+8
and all supported Fedora releases this is the kmod package, and for
CentOS/RHEL 6 it is the module-init-tools package.

Reviewed-by: Minh Diep <mdiep@whamcloud.com>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8724
Closes #9310