fix: remove import mcpp.platform from package_fetcher.cppm

The added import changed the module dependency graph fingerprint,
invalidating BMI cache on macOS CI and exposing a pre-existing
Xcode 16.4 SDK incompatibility during std module precompilation.

Use #if defined(_WIN32) instead of if constexpr for the USERPROFILE
check to avoid changing the module import set.

Author: Sunrisepeak

src/pm/package_fetcher.cppm

@@ -14,7 +14,6 @@ export module mcpp.pm.package_fetcher;
 
 import std;
 import mcpp.config;
-import mcpp.platform;
 import mcpp.pm.compat;
 import mcpp.pm.index_spec;
 import mcpp.xlings;
@@ -614,9 +613,9 @@ Fetcher::resolve_xpkg_path(std::string_view target,
         // reason (xlings subprocess XLINGS_HOME propagation is unreliable).
         if (!std::filesystem::exists(verdir)) {
             const char* xhome = nullptr;
-            if constexpr (mcpp::platform::is_windows) {
-                xhome = std::getenv("USERPROFILE");
-            }
+#if defined(_WIN32)
+            xhome = std::getenv("USERPROFILE");
+#endif
             if (!xhome) xhome = std::getenv("HOME");
             if (xhome) {
                 // xlings stores xpkgs at <home>/.xlings/data/xpkgs/ or