At present, "guix build --source icedove" simply returns the corresponding IceCat tarball. The addition of code from upstream Thunderbird, as well as the rebranding to "IceDove", is currently done within phases.
It would be good to arrange for "guix build --source icedove" to return something more appropriate. I suggest that the first step should add an 'icedove-source' variable that produces an icedove tarball, for now implemented in a way similar to 'icecat-source' and incorporating the functionality of the 'prepare-thunderbird-sources' and 'rename-to-icedove' phases of the 'icedove' package. Note that snippets are unable to do this job, because they are only able to produce tarballs with the same file name and top-level directory name as the main upstream input. In this case, we need to produce a tarball with name and top-level directory 'icedove' starting from an 'icecat' tarball. Mark