bug#43660: icedove: "guix build --source" should produce IceDove source

[Mark H Weaver <mhw@netris.org>]

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