Validate that there are no loose source files in the Tests directory

Sources/PackageLoading/PackageBuilder.swift

@@ -562,8 +562,19 @@ public struct PackageBuilder {
             return []
         }
 
+        // Get the contents of the Tests directory.
+        let testsDirContents = try directoryContents(testsPath)
+
+        // Check that the Tests directory doesn't contain any loose source files.
+        // FIXME: Right now we just check for source files.  We need to decide whether we should check for other kinds of files too.
+        // FIXME: We should factor out the checking for the `LinuxMain.swift` source file.  So ugly...
+        let looseSourceFiles = testsDirContents.filter(isValidSource).filter({ $0.basename.lowercased() != "linuxmain.swift" })
+        guard looseSourceFiles.isEmpty else {
+            throw ModuleError.invalidLayout(.unexpectedSourceFiles(looseSourceFiles.map{ $0.asString }))
+        }
+
         // Create the test modules
-        return try directoryContents(testsPath).filter(shouldConsiderDirectory).flatMap { dir in
+        return try testsDirContents.filter(shouldConsiderDirectory).flatMap { dir in
             return [try createModule(dir, name: dir.basename, isTest: true)]
         }
     }

Tests/PackageLoadingTests/ConventionTests.swift

@@ -554,6 +554,16 @@ class ConventionTests: XCTestCase {
         }
     }
 
+    func testLooseSourceFileInTestsDir() throws {
+        // Loose source file in Tests/
+        let fs = InMemoryFileSystem(emptyFiles:
+            "/Sources/main.swift",
+            "/Tests/source.swift")
+        PackageBuilderTester("LooseSourceFileInTestsDir", in: fs) { result in
+            result.checkDiagnostic("the package has an unsupported layout, unexpected source file(s) found: /Tests/source.swift fix: move the file(s) inside a module")
+        }
+    }
+
     func testManifestTargetDeclErrors() throws {
         // Reference a target which doesn't exist.
         var fs = InMemoryFileSystem(emptyFiles:
@@ -857,6 +867,7 @@ class ConventionTests: XCTestCase {
         ("testCustomTargetDependencies", testCustomTargetDependencies),
         ("testTestTargetDependencies", testTestTargetDependencies),
         ("testInvalidTestTargets", testInvalidTestTargets),
+        ("testLooseSourceFileInTestsDir", testLooseSourceFileInTestsDir),
         ("testManifestTargetDeclErrors", testManifestTargetDeclErrors),
         ("testProducts", testProducts),
         ("testBadProducts", testBadProducts),