Put build artifacts in `debian/build` by default instead of `_build`

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.