Add support for debian overrides

[cpuguy83]

Following up on #396

debbuild has a lot of rules it runs that can be problematic for some projects.
This can be overwritten in the spec at build time for some things by adding build steps that writes to debian/rules, e.g.:

- command: echo "override_dh_dwz:\n\t@" > debian/rules

This is at least a workable escape hatch but it would be nice if we could do better.
We could add something to the spec to do this but this doesn't seem particularly great since its specific to the underlying builder implementation.

Another idea I was thinking about is having an extra statement in our generated debian/rules that calls something like -include dh_overrides.mk.
This would allow users to include a file called dh_overrides.mk in their sources it will be automatically loaded into debian/rules.
If the file doesn't exist then make will ignore it.

[cpuguy83]

cc @adamperlin

[adamperlin]

I agree that adding something to the spec is not the best course of action here as it seems to break the spec abstraction of being distro nonspecific. I do like the idea of adding the -include dh_overrides.mk to allow for overrides if a user specifically adds that file as a source. We will just need to make sure this is well documented, though it will hopefully be a bit of an edge case.