Expose public/ API as submodule (#23345)

* model -> public/model

* plugin -> public/plugin

* public/model/utils -> public/utils

* platform/shared/mlog -> public/shared/mlog

* platform/shared/i18n -> public/shared/i18n

* platform/shared/markdown -> public/shared/markdown

* platform/services/timezones -> public/shared/timezones

* channels/einterfaces -> einterfaces

* expose public/ submodule

* go mod tidy

* .github: cache-dependency-path, setup-go-work

* modules-tidy for public/ too

* remove old gomodtidy

server/channels/api4/api.go

@@ -10,9 +10,9 @@ import (
 	graphql "github.com/graph-gophers/graphql-go"
 	_ "github.com/mattermost/go-i18n/i18n"
 
+	"github.com/mattermost/mattermost-server/server/public/model"
 	"github.com/mattermost/mattermost-server/server/v8/channels/app"
 	"github.com/mattermost/mattermost-server/server/v8/channels/web"
-	"github.com/mattermost/mattermost-server/server/v8/model"
 )
 
 type Routes struct {