go: Check if GO_IMPORT is set in recipe and error if not

go: Check if GO_IMPORT is set in recipe and error if not

Check if the variable GO_IMPORT is
set in the recipe. If not generate an error.
Test building go-helloworld when GO_IMPORT assigned
Test building go-helloworld when GO_IMPORT is not assigned, generate error about GO_IMPORT
Test building any other recipe(e.g bash) when GO_IMPORT is not assigned, generate error about GO_IMPORT
Test creating a GO recipe with recipetool (not affected)
Test selftest test_recipetool_create_go (not affected)
Test selftest test_recipetool_create_go_replace_modules (not affected)

[YOCTO #15763]

CC: Yoann Congal <yoann.congal@smile.fr>
CC: Randy MacLeod <randy.macleod@windriver.com>
Signed-off-by: Christos Gavros <gavrosc@yahoo.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>