Depending on whether the source is a directory or an archive, we strip the
source directory or preserve it, respectively. This change makes it so that
whether the type of the source, it is unpacked at the expected location given
by the IMPORT-PATH of the Go build system.
* guix/build/go-build-system.scm: Add the (ice-9 ftw) module.
(unpack): Add inner procedure to maybe strip the top level directory of an
archive, document it and use it.
