[M5+] add resolver::nix_cmake_scan

tests/nix_cmake_scan_live.cpp

@@ -0,0 +1,57 @@
+// nix-store-gated integration test for resolver::nix_cmake_scan.
+// Realizes nixpkgs#fmt.dev (cheap if already cached) and verifies that
+// nix_cmake_scan picks up its real *-targets.cmake IMPORTED targets.
+
+#include <catch2/catch_test_macros.hpp>
+
+import cargoxx.resolver;
+import cargoxx.exec;
+import cargoxx.util;
+import std;
+
+namespace {
+
+auto network_tests_enabled() -> bool {
+    auto* env = std::getenv("CARGOXX_NETWORK_TESTS");
+    return env != nullptr && std::string_view{env} == "1";
+}
+
+auto realize(std::string_view attr) -> std::optional<std::filesystem::path> {
+    auto r = cargoxx::exec::run(
+        "nix",
+        {"--extra-experimental-features", "nix-command flakes",
+         "build", std::format("nixpkgs#{}", attr),
+         "--no-link", "--print-out-paths"},
+        cargoxx::exec::ExecOptions{
+            .cwd = {},
+            .env_overrides = {},
+            .timeout = std::chrono::seconds{180},
+            .inherit_stdio = false,
+        });
+    if (!r || r->exit_code != 0) {
+        return std::nullopt;
+    }
+    auto path = r->stdout_text;
+    while (!path.empty() && (path.back() == '\n' || path.back() == ' ')) {
+        path.pop_back();
+    }
+    return std::filesystem::path{path};
+}
+
+} // namespace
+
+TEST_CASE("nix_cmake_scan finds fmt's IMPORTED targets",
+          "[resolver][network]") {
+    if (!network_tests_enabled()) {
+        SKIP("CARGOXX_NETWORK_TESTS != 1");
+    }
+    auto p = realize("fmt.dev");
+    REQUIRE(p.has_value());
+
+    auto r = cargoxx::resolver::nix_cmake_scan(*p, "fmt");
+    REQUIRE(r.has_value());
+    REQUIRE(r->find_package == "fmt CONFIG REQUIRED");
+    // fmt-targets.cmake declares fmt::fmt + fmt::fmt-header-only.
+    REQUIRE(std::ranges::find(r->targets, std::string{"fmt::fmt"}) !=
+            r->targets.end());
+}