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/api/api.go

@@ -0,0 +1,57 @@
+package api
+
+import (
+	"github.com/gorilla/mux"
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/internal/api/auth"
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/internal/environment"
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/internal/runner"
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/pkg/logging"
+	"net/http"
+)
+
+var log = logging.GetLogger("api")
+
+const (
+	BasePath         = "/api/v1"
+	HealthPath       = "/health"
+	RunnersPath      = "/runners"
+	EnvironmentsPath = "/execution-environments"
+)
+
+// NewRouter returns a *mux.Router which can be
+// used by the net/http package to serve the routes of our API. It
+// always returns a router for the newest version of our API. We
+// use gorilla/mux because it is more convenient than net/http, e.g.
+// when extracting path parameters.
+func NewRouter(runnerManager runner.Manager, environmentManager environment.Manager) *mux.Router {
+	router := mux.NewRouter()
+	// this can later be restricted to a specific host with
+	// `router.Host(...)` and to HTTPS with `router.Schemes("https")`
+	configureV1Router(router, runnerManager, environmentManager)
+	router.Use(logging.HTTPLoggingMiddleware)
+	return router
+}
+
+// configureV1Router configures a given router with the routes of version 1 of the Poseidon API.
+func configureV1Router(router *mux.Router, runnerManager runner.Manager, environmentManager environment.Manager) {
+	v1 := router.PathPrefix(BasePath).Subrouter()
+	v1.HandleFunc(HealthPath, Health).Methods(http.MethodGet)
+
+	runnerController := &RunnerController{manager: runnerManager}
+	environmentController := &EnvironmentController{manager: environmentManager}
+
+	configureRoutes := func(router *mux.Router) {
+		runnerController.ConfigureRoutes(router)
+		environmentController.ConfigureRoutes(router)
+	}
+
+	if auth.InitializeAuthentication() {
+		// Create new authenticated subrouter.
+		// All routes added to v1 after this require authentication.
+		authenticatedV1Router := v1.PathPrefix("").Subrouter()
+		authenticatedV1Router.Use(auth.HTTPAuthenticationMiddleware)
+		runnerController.ConfigureRoutes(authenticatedV1Router)
+	} else {
+		configureRoutes(v1)
+	}
+}