使用方式：
$ gf mod -h
USAGE
gf mod ARGUMENT
ARGUMENT
path copy all packages with its latest version in Go modules, which does not exist
in GOPATH, to GOPATH. This enables your project using GOPATH building, but you
should have GOPATH environment variable configured.
EXAMPLES
gf mod path
该命令用于将当前go modules包拷贝到GOPATH中，以便使用原始的GOPATH方式开发，往往为了方便调试本地开发的依赖包。