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/environments.go

@@ -0,0 +1,59 @@
+package api
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"github.com/gorilla/mux"
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/internal/environment"
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/internal/runner"
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/pkg/dto"
+	"net/http"
+)
+
+const (
+	executionEnvironmentIDKey = "executionEnvironmentId"
+	createOrUpdateRouteName   = "createOrUpdate"
+)
+
+var ErrMissingURLParameter = errors.New("url parameter missing")
+
+type EnvironmentController struct {
+	manager environment.Manager
+}
+
+func (e *EnvironmentController) ConfigureRoutes(router *mux.Router) {
+	environmentRouter := router.PathPrefix(EnvironmentsPath).Subrouter()
+	specificEnvironmentRouter := environmentRouter.Path(fmt.Sprintf("/{%s:[0-9]+}", executionEnvironmentIDKey)).Subrouter()
+	specificEnvironmentRouter.HandleFunc("", e.createOrUpdate).Methods(http.MethodPut).Name(createOrUpdateRouteName)
+}
+
+// createOrUpdate creates/updates an execution environment on the executor.
+func (e *EnvironmentController) createOrUpdate(writer http.ResponseWriter, request *http.Request) {
+	req := new(dto.ExecutionEnvironmentRequest)
+	if err := json.NewDecoder(request.Body).Decode(req); err != nil {
+		writeBadRequest(writer, err)
+		return
+	}
+
+	id, ok := mux.Vars(request)[executionEnvironmentIDKey]
+	if !ok {
+		writeBadRequest(writer, fmt.Errorf("could not find %s: %w", executionEnvironmentIDKey, ErrMissingURLParameter))
+		return
+	}
+	environmentID, err := runner.NewEnvironmentID(id)
+	if err != nil {
+		writeBadRequest(writer, fmt.Errorf("could not update environment: %w", err))
+		return
+	}
+	created, err := e.manager.CreateOrUpdate(environmentID, *req)
+	if err != nil {
+		writeInternalServerError(writer, err, dto.ErrorUnknown)
+	}
+
+	if created {
+		writer.WriteHeader(http.StatusCreated)
+	} else {
+		writer.WriteHeader(http.StatusNoContent)
+	}
+}