klaytn package management is migrated from Dep to Go Modules

[miloveme]

how about migrate klaytn package management from Dep to Go Moudles

Migration Guide

1. Run go version and make sure you're using version 11 or later.
2. Move your code outside of GOPATH or set export GO111MODULE=on.
3. go mod init [module path]: This will import dependencies from Gopkg.lock.
4. go mod tidy: This will remove unnecessary imports, and add indirect ones.
5. rm -rf vendor/: Optional step to delete your vendor folder.
6. go build: Do a test build to see if it works.
7. rm -f Gopkg.lock Gopkg.toml: Delete the obsolete files used for Dep.

[miloveme]

In klaytn project, Makefile should be modified to use Go Module
Makefile :
...
build/env.sh go run build/ci.go install ./cmd/kcn
...
all of 'build/env.sh' is removed because Go Module doesn't need go environment

[miloveme]

If you want to keep your vendor folder:
do not 7 step mentioned and do following step instead of 7 step
7. Run go mod vendor to copy your dependencies into the vendor folder.
8. Run go build -mod=vendor to ensure go build uses your vendor folder.