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

internal/runner/nomad_environment_storage.go

@@ -0,0 +1,75 @@
+package runner
+
+import (
+	"sync"
+)
+
+// NomadEnvironmentStorage is an interface for storing Nomad environments.
+type NomadEnvironmentStorage interface {
+	// List returns all environments stored in this storage.
+	List() []*NomadEnvironment
+
+	// Add adds an environment to the storage.
+	// It overwrites the old environment if one with the same id was already stored.
+	Add(environment *NomadEnvironment)
+
+	// Get returns an environment from the storage.
+	// Iff the environment does not exist in the store, ok will be false.
+	Get(id EnvironmentID) (environment *NomadEnvironment, ok bool)
+
+	// Delete deletes the environment with the passed id from the storage. It does nothing if no environment with the id
+	// is present in the storage.
+	Delete(id EnvironmentID)
+
+	// Length returns the number of currently stored environments in the storage.
+	Length() int
+}
+
+// localNomadEnvironmentStorage stores NomadEnvironment objects in the local application memory.
+type localNomadEnvironmentStorage struct {
+	sync.RWMutex
+	environments map[EnvironmentID]*NomadEnvironment
+}
+
+// NewLocalNomadEnvironmentStorage responds with an empty localNomadEnvironmentStorage.
+// This implementation stores the data thread-safe in the local application memory.
+func NewLocalNomadEnvironmentStorage() *localNomadEnvironmentStorage {
+	return &localNomadEnvironmentStorage{
+		environments: make(map[EnvironmentID]*NomadEnvironment),
+	}
+}
+
+func (s *localNomadEnvironmentStorage) List() []*NomadEnvironment {
+	s.RLock()
+	defer s.RUnlock()
+	values := make([]*NomadEnvironment, 0, len(s.environments))
+	for _, v := range s.environments {
+		values = append(values, v)
+	}
+	return values
+}
+
+func (s *localNomadEnvironmentStorage) Add(environment *NomadEnvironment) {
+	s.Lock()
+	defer s.Unlock()
+	s.environments[environment.ID()] = environment
+}
+
+func (s *localNomadEnvironmentStorage) Get(id EnvironmentID) (environment *NomadEnvironment, ok bool) {
+	s.RLock()
+	defer s.RUnlock()
+	environment, ok = s.environments[id]
+	return
+}
+
+func (s *localNomadEnvironmentStorage) Delete(id EnvironmentID) {
+	s.Lock()
+	defer s.Unlock()
+	delete(s.environments, id)
+}
+
+func (s *localNomadEnvironmentStorage) Length() int {
+	s.RLock()
+	defer s.RUnlock()
+	return len(s.environments)
+}