[wip] use go modules

[sharanyad]

Summary

moving away from godep for dependency management and using go modules.

Implementation details

from agent/

go mod init
go mod tidy
rm -rf vendor
go mod vendor

Testing

New tests cover the changes:

Description for the changelog

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sharanyad]

build failure in windows:

+go build -installsuffix cgo -a -ldflags ' -s' -o out/amazon-ecs-agent ./agent/
# github.com/aws/amazon-ecs-agent/agent/vendor/github.com/docker/docker/pkg/system
agent/vendor/github.com/docker/docker/pkg/system/filesys_windows.go:113:24: cannot use uintptr(unsafe.Pointer(&sd[0])) (type uintptr) as type *"github.com/aws/amazon-ecs-agent/agent/vendor/golang.org/x/sys/windows".SECURITY_DESCRIPTOR in assignment
Makefile:44: recipe for target 'xplatform-build' failed
make: *** [xplatform-build] Error 2
The command "make xplatform-build" exited with 2.

[sharanyad]

golang/go#34610 - x/sys API changes broke windows

needs update to docker/docker package to consume the latest x/sys/windows

[mythri-garaga]

WIP #2576