Put build artifacts in `debian/.build/upstream` by default instead of `_build`
This is a re-submission of !25 (merged) with debian/build changed to debian/.build/upstream.
The previous change was reverted by Guilliem reverted in b01d3fbd. His justification was that his proposal from 2020 (https://lists.debian.org/debian-dpkg/2020/03/msg00003.html) to use debian/.build/upstream is soon going to supersede this, and which would risk the path changing twice in vain.
Most Debian build systems place the build artifacts in a subdirectory of
debian/. Using _build in the project root lead to most Go packages
adding _build to their .gitignore even though upstream sources should
ideally stay pristine and all modifications be done only in debian/.
Note that this has little immediate effect on Go packages, as most of them
are using a debian/rules template from dh-make-golang which defines
explicitly a build path. Those need to be removed before this new default
setting will have an effect.
Note that these two changes should stay aligned: