Add option to set custom proto path

PluginExamples/Package.swift

@@ -47,13 +47,23 @@ let package = Package(
                 .plugin(name: "SwiftProtobufPlugin", package: "swift-protobuf")
             ]
         ),
+        .target(
+            name: "CustomProtoPath",
+            dependencies: [
+                .product(name: "SwiftProtobuf", package: "swift-protobuf")
+            ],
+            plugins: [
+                .plugin(name: "SwiftProtobufPlugin", package: "swift-protobuf")
+            ]
+        ),
         .testTarget(
             name: "ExampleTests",
             dependencies: [
                 .target(name: "Simple"),
                 .target(name: "Nested"),
                 .target(name: "Import"),
                 .target(name: "AccessLevelOnImport"),
+                .target(name: "CustomProtoPath"),
             ]
         ),
     ],

PluginExamples/Sources/CustomProtoPath/empty.swift

@@ -0,0 +1,3 @@
+/// DO NOT DELETE.
+///
+/// We need to keep this file otherwise the plugin is not running.

PluginExamples/Sources/CustomProtoPath/protos/Bar/Bar.proto

@@ -0,0 +1,5 @@
+syntax = "proto3";
+
+message Bar {
+  string name = 1;
+}

PluginExamples/Sources/CustomProtoPath/protos/Foo/Foo.proto

@@ -0,0 +1,7 @@
+syntax = "proto3";
+
+import "Bar/Bar.proto";
+
+message Foo {
+  Bar bar = 1;
+}

PluginExamples/Sources/CustomProtoPath/protos/Main.proto

@@ -0,0 +1,9 @@
+syntax = "proto3";
+
+import "Bar/Bar.proto";
+import "Foo/Foo.proto";
+
+message Main {
+  Bar bar = 1;
+  Foo foo = 2;
+}

PluginExamples/Sources/CustomProtoPath/swift-protobuf-config.json

@@ -0,0 +1,13 @@
+{
+    "invocations": [
+        {
+            "protoFiles": [
+                "Bar/Bar.proto",
+                "Foo/Foo.proto",
+                "Main.proto"
+            ],
+            "visibility": "public",
+            "protoPath": "protos"
+        }
+    ]
+}

PluginExamples/Sources/ExampleTests/ExampleTests.swift

@@ -1,3 +1,4 @@
+import CustomProtoPath
 import Import
 import Nested
 import Simple
@@ -15,7 +16,16 @@ final class ExampleTests: XCTestCase {
     }
 
     func testImport() {
-        let foo = Foo.with { $0.bar = .with { $0.name = "Bar" } }
+        let foo = Import.Foo.with { $0.bar = .with { $0.name = "Bar" } }
         XCTAssertEqual(foo.bar.name, "Bar")
     }
+
+    func testCustomProtoPath() {
+        let main = CustomProtoPath.Main.with {
+            $0.bar = .with { $0.name = "Bar" }
+            $0.foo = .with { $0.bar = .with { $0.name = "BarInFoo" } }
+        }
+        XCTAssertEqual(main.bar.name, "Bar")
+        XCTAssertEqual(main.foo.bar.name, "BarInFoo")
+    }
 }

Plugins/SwiftProtobufPlugin/plugin.swift

@@ -77,6 +77,9 @@ struct SwiftProtobufPlugin {
             }
 
             /// An array of paths to `.proto` files for this invocation.
+            ///
+            /// If the `protoPath` parameter is specified, the files must be specified
+            /// relative to that directory. Otherwise, relative to the target source directory.
             var protoFiles: [String]
             /// The visibility of the generated files.
             var visibility: Visibility?
@@ -86,6 +89,15 @@ struct SwiftProtobufPlugin {
             var implementationOnlyImports: Bool?
             /// Whether import statements should be preceded with visibility.
             var useAccessLevelOnImports: Bool?
+            /// Overrides the base directory used to find protobuf files.
+            ///
+            /// This must be specified as a path relative to the target source directory.
+            /// For example, if you are storing the protofiles at `MyLibrary/Sources/MyLib/protos`,
+            /// you should specify `protos` as the value for this parameter.
+            ///
+            /// If you have multiple subdirectories you wish to include,
+            /// you should specify multiple `invocations` instead.
+            var protoPath: String?
         }
 
         /// The path to the `protoc` binary.
@@ -171,10 +183,15 @@ struct SwiftProtobufPlugin {
             "--swift_out=\(outputDirectory)",
         ]
 
-        // We need to add the target directory as a search path since we require the user to specify
-        // the proto files relative to it.
+        let protoDirectory =
+            if let protoPath = invocation.protoPath {
+                directory.appending(protoPath)
+            } else {
+                directory
+            }
+
         protocArgs.append("-I")
-        protocArgs.append("\(directory)")
+        protocArgs.append("\(protoDirectory)")
 
         // Add the visibility if it was set
         if let visibility = invocation.visibility {
@@ -202,7 +219,7 @@ struct SwiftProtobufPlugin {
         for var file in invocation.protoFiles {
             // Append the file to the protoc args so that it is used for generating
             protocArgs.append("\(file)")
-            inputFiles.append(directory.appending(file))
+            inputFiles.append(protoDirectory.appending(file))
 
             // The name of the output file is based on the name of the input file.
             // We validated in the beginning that every file has the suffix of .proto