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/health.go

@@ -0,0 +1,12 @@
+package api
+
+import (
+	"net/http"
+)
+
+// Health handles the health route.
+// It responds that the server is alive.
+// If it is not, the response won't reach the client.
+func Health(writer http.ResponseWriter, _ *http.Request) {
+	writer.WriteHeader(http.StatusNoContent)
+}