go: do not unpack go modules by default

Before this change, they were unpacked into ${WRKDIR}, but the actual build
process started from the zip archives again, leading to two copies of the
files on disk; even worse, you could assume the extracted sources were
used and could be patched.

Now, only the main tar.gz will be extracted into ${WRKDIR} and other
files need to be added to GO_MODULE_EXTRACT to be extracted.

Adapt the couple packages that already patch go sources, or need other
files extracted.

With help from bsiegert, thanks!

(wiz)