Add package visibility modifier (grpc#1674)

Author: gjcairo

Plugins/GRPCSwiftPlugin/plugin.swift

@@ -53,6 +53,8 @@ struct GRPCSwiftPlugin {
         case `internal`
         /// The generated files should have `public` access level.
         case `public`
+        /// The generated files should have `package` access level.
+        case `package`
       }
 
       /// An array of paths to `.proto` files for this invocation.

Sources/protoc-gen-grpc-swift/options.swift

@@ -40,13 +40,16 @@ final class GeneratorOptions {
   enum Visibility: String {
     case `internal` = "Internal"
     case `public` = "Public"
+    case `package` = "Package"
 
     var sourceSnippet: String {
       switch self {
       case .internal:
         return "internal"
       case .public:
         return "public"
+      case .package:
+        return "package"
       }
     }
   }

docs/plugin.md

@@ -27,7 +27,7 @@ when invoking `protoc`.
 
 The **Visibility** option determines the access control for generated code.
 
-- **Possible values:** Public, Internal
+- **Possible values:** Public, Internal, Package
 - **Default value:** Internal
 
 ### Server