[SRU {B,D,E,F}] LP#1850958: pull dkms packages from librarian

[Andy Whitcroft]

When we build a kernel we capture the current dkms package versions.
This allows us to keep the versions syncronised in an entire SRU cycle
between the master and derivative kernels.  It also allows us to maintain
the same versions on respin.  This suffers from the issue that they may
expire from the archive pool if the dkms packages are revved.  Use the
launchpad librarian as an alternative source of these packages.  Use if
first as they should never be removed (unlike the pool).  Retain the
pool search as this will work in environments where the librarian is not
available (no internet connectivity).

Proposing for SRU to bionic, disco, eoan, and application to focal.

-apw