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/bundle_info_test.go

@@ -0,0 +1,30 @@
+package plugin
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestBundleInfoForPath(t *testing.T) {
+	dir, err := ioutil.TempDir("", "mm-plugin-test")
+	require.NoError(t, err)
+	defer os.RemoveAll(dir)
+
+	path := filepath.Join(dir, "plugin.json")
+	f, err := os.Create(path)
+	require.NoError(t, err)
+	_, err = f.WriteString(`{"id": "foo"}`)
+	f.Close()
+	require.NoError(t, err)
+
+	info := BundleInfoForPath(dir)
+	assert.Equal(t, info.Path, dir)
+	assert.NotNil(t, info.Manifest)
+	assert.Equal(t, info.ManifestPath, path)
+	assert.Nil(t, info.ManifestError)
+}