Restructure project

We previously didn't really had any structure in our project apart
from creating a new folder for each package in our project root.
Now that we have accumulated some packages, we use the well-known
Golang project layout in order to clearly communicate our intent
with packages. See https://github.com/golang-standards/project-layout

tests/constants.go

@@ -3,7 +3,7 @@ package tests
 import (
 	"errors"
 	nomadApi "github.com/hashicorp/nomad/api"
-	"gitlab.hpi.de/codeocean/codemoon/poseidon/api/dto"
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/pkg/dto"
 	"time"
 )