fiber: 🐛 [Bug]: Naming of routes works wrong after mount

Bug Description

Naming of routes after mounting sub-apps into root app works wrong. All routes turn into flat map of routes, but expected that they all separately work without any changing in original sub-apps. Also there is no way to set names of sub-apps (but it could be great, as groups have this feature).

How to Reproduce

Steps to reproduce the behavior:

1. Create few sub-apps
2. Setup some routes with names
3. Add sub-apps to root app
4. Try to use names in handlers or in main.go to find any Routes

Expected Behavior

Expected, that I can use naming in some sub-apps and it works. And after that if I mount sub-app to root-app, naming also works, but with correct prefixes (of paths and of names, like it works with groups).

I added some test, that could be placed into the end of app_test.go. Check if the proposal behaviour is correct.

Fiber Version

v2.50.0

Code Snippet (optional)

func Test_Mount_Route_Names(t *testing.T) {
	// create sub-app with 2 handlers:
	subApp1 := New()
	subApp1.Name("app1.") // add some name to the app
	subApp1.Get("/users", func(c *Ctx) error {
		url, err := c.GetRouteURL("add-user", Map{})
		utils.AssertEqual(t, nil, err)
		utils.AssertEqual(t, "/app1/users", url, "handler: app1.add-user") // the prefix is /app1 because of the mount
		// if subApp1 is not mounted, expected url just /users
		return nil
	}).Name("get-users")
	subApp1.Post("/users", func(c *Ctx) error {
		route := c.App().GetRoute("get-users")
		utils.AssertEqual(t, MethodGet, route.Method, "handler: app1.get-users method")
		utils.AssertEqual(t, "/app1/users", route.Path, "handler: app1.get-users path")
		return nil
	}).Name("add-user")

	// create sub-app with 2 handlers inside a group:
	subApp2 := New()
	subApp2.Name("app2.") // add some name to the app
	app2Grp := subApp2.Group("/users").Name("users.")
	app2Grp.Get("", nil).Name("get")
	app2Grp.Post("", nil).Name("add")

	// put both sub-apps into root app
	rootApp := New()
	rootApp.Mount("/app1", subApp1)
	rootApp.Mount("/app2", subApp2)
	rootApp.startupProcess()

	// take route directly from sub-app
	route := subApp1.GetRoute("get-users")
	utils.AssertEqual(t, MethodGet, route.Method)
	utils.AssertEqual(t, "/users", route.Path)
	route = subApp1.GetRoute("add-user")
	utils.AssertEqual(t, MethodPost, route.Method)
	utils.AssertEqual(t, "/users", route.Path)

	// take route directly from sub-app with group
	route = subApp2.GetRoute("users.get")
	utils.AssertEqual(t, MethodGet, route.Method)
	utils.AssertEqual(t, "/users", route.Path)
	route = subApp2.GetRoute("users.add")
	utils.AssertEqual(t, MethodPost, route.Method)
	utils.AssertEqual(t, "/users", route.Path)

	// take route from root app (using names of sub-apps)
	route = rootApp.GetRoute("app1.add-user")
	utils.AssertEqual(t, MethodPost, route.Method)
	utils.AssertEqual(t, "/app1/users", route.Path)
	route = rootApp.GetRoute("app2.users.add")
	utils.AssertEqual(t, MethodPost, route.Method)
	utils.AssertEqual(t, "/app2/users", route.Path)

	// GetRouteURL inside handler
	req := httptest.NewRequest(MethodGet, "/app1/users", nil)
	_, _ = rootApp.Test(req)

	// ctx.App().GetRoute() inside handler
	req = httptest.NewRequest(MethodPost, "/app1/users", nil)
	_, _ = rootApp.Test(req)
}

Checklist:

• I agree to follow Fiber’s Code of Conduct.
• I have checked for existing issues that describe my problem prior to opening this one.
• I understand that improperly formatted bug reports may be closed without explanation.