PLT-7407: Back-end plugin mechanism (#7177)

* begin backend plugin wip

* flesh out rpcplugin. everything done except for minor supervisor stubs

* done with basic plugin infrastructure

* simplify tests

* remove unused test lines

plugin/pluginenv/search_path.go

@@ -0,0 +1,32 @@
+package pluginenv
+
+import (
+	"io/ioutil"
+	"path/filepath"
+
+	"github.com/mattermost/platform/plugin"
+)
+
+// Performs a full scan of the given path.
+//
+// This function will return info for all subdirectories that appear to be plugins (i.e. all
+// subdirectories containing plugin manifest files, regardless of whether they could actually be
+// parsed).
+//
+// Plugins are found non-recursively and paths beginning with a dot are always ignored.
+func ScanSearchPath(path string) ([]*plugin.BundleInfo, error) {
+	files, err := ioutil.ReadDir(path)
+	if err != nil {
+		return nil, err
+	}
+	var ret []*plugin.BundleInfo
+	for _, file := range files {
+		if !file.IsDir() || file.Name()[0] == '.' {
+			continue
+		}
+		if info := plugin.BundleInfoForPath(filepath.Join(path, file.Name())); info.ManifestPath != "" {
+			ret = append(ret, info)
+		}
+	}
+	return ret, nil
+}