Example for Rosdep yaml using source [closed]

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I refer to this rosdep guide, where it shows the possible methods of getting a system dependency.

I get that for pip, it would be something like this:

minimalmodbus:
  ubuntu:
    pip:
      packages: [minimalmodbus]

and for apt:

protobuf:
  ubuntu:
    xenial:
      packages: [protobuf]

but what about source? Is there an example?

I have a repo with a dependency that is another repo which I have packaged as a debian pkg and stored on Github Releases. Though I have included the dependency in package.xml, it's not in the rosdep database or apt or pip, so it's quite inaccessible. I am looking for a way of downloading that package from Github Releases, hopefully through rosdep, otherwise, Im open to more ideas.