env: Move each component into a separate struct

This commit cleans up the distillery code by making each component a
distinct struct. Each of these components is also returned by by a new
Component() function that replaces the Stacks() function.

env/stack_web.go

@@ -2,14 +2,28 @@ package env
 
 import "github.com/FAU-CDI/wisski-distillery/internal/stack"
 
-func (dis *Distillery) WebStack() stack.Installable {
-	return dis.asCoreStack("web", stack.Installable{
+// WebComponent represents the 'web' layer belonging to a distillery
+type WebComponent struct {
+	dis *Distillery
+}
+
+// Web returns the WebComponent belonging to this distillery
+func (dis *Distillery) Web() WebComponent {
+	return WebComponent{dis: dis}
+}
+
+func (WebComponent) Name() string {
+	return "web"
+}
+
+func (web WebComponent) Stack() stack.Installable {
+	return web.dis.makeComponentStack(web, stack.Installable{
 		EnvFileContext: map[string]string{
-			"DEFAULT_HOST": dis.Config.DefaultDomain,
+			"DEFAULT_HOST": web.dis.Config.DefaultDomain,
 		},
 	})
 }
 
-func (dis *Distillery) WebStackPath() string {
-	return dis.WebStack().Dir
+func (web WebComponent) Path() string {
+	return web.Stack().Dir
 }