Correctly mark executable targets, explicitly ignore files that are not used by SwiftPM

Package.swift

@@ -22,13 +22,17 @@ let package = Package(
     .package(url: "https://github.com/apple/swift-docc-plugin", from: "1.0.0"),
   ],
   targets: [
-    .target(name: "SwiftProtobuf"),
+    .target(name: "SwiftProtobuf",
+            exclude: ["CMakeLists.txt"]),
     .target(name: "SwiftProtobufPluginLibrary",
-            dependencies: ["SwiftProtobuf"]),
-    .target(name: "protoc-gen-swift",
-            dependencies: ["SwiftProtobufPluginLibrary", "SwiftProtobuf"]),
-    .target(name: "Conformance",
-            dependencies: ["SwiftProtobuf"]),
+            dependencies: ["SwiftProtobuf"],
+            exclude: ["CMakeLists.txt"]),
+    .executableTarget(name: "protoc-gen-swift",
+            dependencies: ["SwiftProtobufPluginLibrary", "SwiftProtobuf"],
+            exclude: ["CMakeLists.txt"]),
+    .executableTarget(name: "Conformance",
+            dependencies: ["SwiftProtobuf"],
+            exclude: ["failure_list_swift.txt", "text_format_failure_list_swift.txt"]),
     .testTarget(name: "SwiftProtobufTests",
                 dependencies: ["SwiftProtobuf"]),
     .testTarget(name: "SwiftProtobufPluginLibraryTests",