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/public/plugin/plugintest/doc.go

@@ -0,0 +1,11 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+// The plugintest package provides mocks that can be used to test plugins.
+//
+// The mocks are created using testify's mock package:
+// https://godoc.org/github.com/stretchr/testify/mock
+//
+// If you need to import the mock package, you can import it with
+// "github.com/mattermost/mattermost-server/server/public/plugin/plugintest/mock".
+package plugintest