Refactor Package structure

This commit cleans up the package structure, to make two new top-level
packages `internal` (for internal-use packages) and `pkg` (for general
shared utility code).

internal/env/runtime.go

@@ -0,0 +1,8 @@
+package env
+
+import "path/filepath"
+
+// RuntimeDir returns the path to the runtime directory
+func (dis *Distillery) RuntimeDir() string {
+	return filepath.Join(dis.Config.DeployRoot, "runtime")
+}