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/constants_test.go

@@ -0,0 +1,9 @@
+package runner
+
+import "gitlab.hpi.de/codeocean/codemoon/poseidon/tests"
+
+const (
+	defaultEnvironmentID     = EnvironmentID(tests.DefaultEnvironmentIDAsInteger)
+	anotherEnvironmentID     = EnvironmentID(tests.AnotherEnvironmentIDAsInteger)
+	defaultInactivityTimeout = 0
+)