package/go: implement go modules integration

package/go: implement go modules integration

The Go compiler needs to know the "import path" to the root of package
source repositories. Previously, this was done by creating a fake
_gopath in the build directory and symlinking the package source into
that path.

Go has deprecated the GOPATH mechanism in favor of a new approach -
Modules - which specifies the root import path (and dependencies) in a
"go.mod" file. This commit moves Buildroot to use the new go.mod
approach, which requires:

 - Passing GO111MODULE=on when building host or target Go packages.

 - Passing GOPROXY=off and -mod=vendor to prevent the Go module system
   from downloading by itself sources from the Internet. We currently
   only support Go packages that have all their dependencies in their
   source tree in "vendor" directories.

 - Specifying a <pkg>_GOMOD variable, which is used both to create a
   minimal go.mod file in the package source tree if it exists, and to
   invoke the right build targets. Indeed, all elements in
   <pkg>_BUILD_TARGETS are now relative to <pkg>_GOMOD.

Reference: https://github.com/golang/go/wiki/Modules

Signed-off-by: Christian Stewart <christian@paral.in>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>