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.go

@@ -0,0 +1,20 @@
+package plugin
+
+type BundleInfo struct {
+	Path string
+
+	Manifest      *Manifest
+	ManifestPath  string
+	ManifestError error
+}
+
+// Returns bundle info for the given path. The return value is never nil.
+func BundleInfoForPath(path string) *BundleInfo {
+	m, mpath, err := FindManifest(path)
+	return &BundleInfo{
+		Path:          path,
+		Manifest:      m,
+		ManifestPath:  mpath,
+		ManifestError: err,
+	}
+}