task/go: restructure go targets

Created by: sylv-io

Enable go modules globally. If possible, use go modules to install go binaries. (go:install) However, u-root does not support modules at runtime and requires each package inside GOPATH. Use go:get and go:checkout to get relevant packages.

This commit also fixes concurrency issues while fetching multiple packages at once.

Signed-off-by: Marcello Sylvester Bauer sylv@sylv.io