Consider excludes while walking Tests

Fixtures/Miscellaneous/ExcludeDiagnostic4/Package.swift

@@ -0,0 +1,6 @@
+import PackageDescription
+
+let package = Package(
+    name: "FooPackage",
+    exclude: ["Tests/Fixtures"]
+)

Fixtures/Miscellaneous/ExcludeDiagnostic4/Sources/main.swift

@@ -0,0 +1 @@
+print("hello")

Fixtures/Miscellaneous/ExcludeDiagnostic5/Package.swift

@@ -0,0 +1,6 @@
+import PackageDescription
+
+let package = Package(
+    name: "FooPackage",
+    exclude: ["Tests"]
+)

Fixtures/Miscellaneous/ExcludeDiagnostic5/Sources/main.swift

@@ -0,0 +1 @@
+print("hello")

Sources/Transmute/Package+testModules.swift

@@ -13,7 +13,10 @@ import Utility
 
 extension Package {
     func testModules() throws -> [TestModule] {
-        return walk(path, "Tests", recursively: false).filter{ $0.isDirectory }.flatMap { dir in
+        let testsPath = Path.join(path, "Tests")
+        //Don't try to walk Tests if it is in excludes
+        if testsPath.isDirectory && excludes.contains(testsPath) { return [] }
+        return walk(testsPath, recursively: false).filter(shouldConsiderDirectory).flatMap { dir in
             if let sources = try? self.sourcify(dir) {
                 return TestModule(basename: dir.basename, sources: sources)
             } else {

Tests/Functional/TestMiscellaneous.swift

@@ -69,6 +69,26 @@ class MiscellaneousTestCase: XCTestCase {
             XCTAssertNoSuchPath(prefix, "App", ".build", "debug", "some")
         }
     }
+
+    func testManifestExcludes4() {
+
+        // exclude directory is inside Tests folder (Won't build without exclude)
+
+        fixture(name: "Miscellaneous/ExcludeDiagnostic4") { prefix in
+            XCTAssertBuilds(prefix)
+            XCTAssertFileExists(prefix, ".build", "debug", "FooPackage.swiftmodule")
+        }
+    }
+
+    func testManifestExcludes5() {
+
+        // exclude directory is Tests folder (Won't build without exclude)
+
+        fixture(name: "Miscellaneous/ExcludeDiagnostic5") { prefix in
+            XCTAssertBuilds(prefix)
+            XCTAssertFileExists(prefix, ".build", "debug", "FooPackage.swiftmodule")
+        }
+    }
 
     func testTestDependenciesSimple() {
     #if false

Tests/Functional/TestValidLayouts.swift

@@ -163,6 +163,8 @@ extension MiscellaneousTestCase {
             ("testManifestExcludes1", testManifestExcludes1),
             ("testManifestExcludes2", testManifestExcludes2),
             ("testManifestExcludes3", testManifestExcludes3),
+            ("testManifestExcludes4", testManifestExcludes4),
+            ("testManifestExcludes5", testManifestExcludes5),
             ("testTestDependenciesSimple", testTestDependenciesSimple),
             ("testTestDependenciesComplex", testTestDependenciesComplex),
             ("testPassExactDependenciesToBuildCommand", testPassExactDependenciesToBuildCommand),