This refactoring work is to prepare for adding a icedove-l10n package, which
will reuse that source.
* gnu/packages/gnuzilla.scm (comm-source->locales+changset): New procedure.
(thunderbird-source): Rename to...
(thunderbird-comm-source): ... here.
(icedove-source): New variable.
(icedove) [source]: Use it.
[phases]: Remove the prepare-thunderbird-sources and rename-to-icedove phases.