Moving markdown to corelibs (#16916)

* Moving markdown to corelibs

* Fix golangci-lint problem

* Renaming corelibs to pkg

* Fixing golangci-lint

* Renaming from pkg to shared

* Fixing gofmt

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/post_metadata.go

@@ -18,9 +18,9 @@ import (
 
 	"github.com/mattermost/mattermost-server/v5/model"
 	"github.com/mattermost/mattermost-server/v5/services/cache"
+	"github.com/mattermost/mattermost-server/v5/shared/markdown"
 	"github.com/mattermost/mattermost-server/v5/shared/mlog"
 	"github.com/mattermost/mattermost-server/v5/utils/imgutils"
-	"github.com/mattermost/mattermost-server/v5/utils/markdown"
 )
 
 type linkMetadataCache struct {