Update mattermost library (#2152)

* Update mattermost library

* Fix linting

vendor/github.com/hashicorp/go-plugin/discover.go

@@ -0,0 +1,31 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+
+package plugin
+
+import (
+	"path/filepath"
+)
+
+// Discover discovers plugins that are in a given directory.
+//
+// The directory doesn't need to be absolute. For example, "." will work fine.
+//
+// This currently assumes any file matching the glob is a plugin.
+// In the future this may be smarter about checking that a file is
+// executable and so on.
+//
+// TODO: test
+func Discover(glob, dir string) ([]string, error) {
+	var err error
+
+	// Make the directory absolute if it isn't already
+	if !filepath.IsAbs(dir) {
+		dir, err = filepath.Abs(dir)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return filepath.Glob(filepath.Join(dir, glob))
+}