Fetch dependency of linux dbgsym package when downloading kernel packages

[pzakha]

The idea for the linux-pkg's linux-kernel-* packages, is that we will store in S3 all of the linux-kernel packages required to build the Delphix Appliance, without relying on the linux-package-mirror snapshot that is passed to appliance-build to contain them.

Some linux-kernel packages have dependencies on other linux-kernel packages. It seems that in some cases the dbgsym package has dependencies on another dbgsym package with a slightly different naming convention, and we were not downloading it.

Here's an example:
linux-image-5.4.0-48-generic-dbgsym depends on linux-image-unsigned-5.4.0-48-generic-dbgsym.

Note that we've already had some logic to deal with a similar issue that we had with the "tools" and "headers" packages, so this just extends the same logic to the dbgsym package.

Testing

build-kernel: http://selfservice.jenkins.delphix.com/job/devops-gate/job/master/job/linux-pkg/job/master/job/build-kernel/job/pre-push/3/console
List of debs produced (i.e. downloaded) by the linux-kernel-generic package:

linux-headers-5.4.0-48-generic_5.4.0-48.52~18.04.1_amd64.deb
linux-hwe-5.4-headers-5.4.0-48_5.4.0-48.52~18.04.1_all.deb
linux-hwe-5.4-tools-5.4.0-48_5.4.0-48.52~18.04.1_amd64.deb
linux-image-5.4.0-48-generic-dbgsym_5.4.0-48.52~18.04.1_amd64.ddeb
linux-image-5.4.0-48-generic_5.4.0-48.52~18.04.1_amd64.deb
linux-image-unsigned-5.4.0-48-generic-dbgsym_5.4.0-48.52~18.04.1_amd64.ddeb
linux-modules-5.4.0-48-generic_5.4.0-48.52~18.04.1_amd64.deb
linux-tools-5.4.0-48-generic_5.4.0-48.52~18.04.1_amd64.deb