test: two builds of one module are refused before anything compiles

The fixture is P -> M and P -> A -> M, where M's own flag --wide
changes a struct it exports. Loaded with --wide --split, P gives M its
args without --wide while A passes them on whole: Build(P) has to fail,
name the module and both paths, and leave no bin/ anywhere. Loaded with
--wide alone, both paths give M the same list, there is one M, and the
build has to go through. Before the fix the first case built cleanly.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/DuplicateModule/fixture/a/interfaces/A.cppm

@@ -0,0 +1,6 @@
+export module A;
+import std;
+import M;
+export std::int32_t ASize() {
+    return Size();
+}

tests/DuplicateModule/fixture/a/project.cpp

@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: LGPL-3.0-only
+// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+import std;
+import Crafter.Build;
+namespace fs = std::filesystem;
+using namespace Crafter;
+
+// Loads M with the default, so M gets whatever A was given.
+extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> args) {
+    Configuration* m = LocalProject({ .projectFile = "../m/project.cpp" });
+
+    Configuration cfg;
+    cfg.path = "./";
+    cfg.name = "A";
+    cfg.outputName = "A";
+    cfg.type = ConfigurationType::LibraryStatic;
+    ApplyStandardArgs(cfg, args);
+    cfg.dependencies = { m };
+    std::array<fs::path, 1> ifaces = { "interfaces/A" };
+    std::array<fs::path, 0> impls = {};
+    cfg.GetInterfacesAndImplementations(ifaces, impls);
+    return cfg;
+}