go-mod.bbclass: Set GO_MOD_CACHE_DIR

Set the GO_MOD_CACHE_DIR variable and move the location of the module
cache to enable the use of the go module fetchers for downloading and
unpacking module dependencies to the module cache.
Also, clean out the module cache before unpacking.

(From OE-Core rev: 287daff0bbd877990ce272e10601d4540d71f239)

Signed-off-by: Christian Lindeberg <christian.lindeberg@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes-recipe/go-mod.bbclass

@@ -22,9 +22,9 @@ GOBUILDFLAGS:append = " -modcacherw"
 
 inherit go
 
+export GOMODCACHE = "${S}/pkg/mod"
+GO_MOD_CACHE_DIR = "${@os.path.relpath(d.getVar('GOMODCACHE'), d.getVar('WORKDIR'))}"
+do_unpack[cleandirs] += "${GOMODCACHE}"
+
 GO_WORKDIR ?= "${GO_IMPORT}"
 do_compile[dirs] += "${B}/src/${GO_WORKDIR}"
-
-export GOMODCACHE = "${B}/.mod"
-
-do_compile[cleandirs] += "${B}/.mod"