fix: add missing import mcpp.platform in package_fetcher.cppm

Also replace raw WIFEXITED/WEXITSTATUS with
platform::process::extract_exit_code() in xlings.cppm.

Author: Sunrisepeak

src/pm/package_fetcher.cppm

@@ -14,6 +14,7 @@ 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;

src/xlings.cppm

@@ -622,10 +622,8 @@ int install_with_progress(const Env& env, std::string_view target,
         // Use std::system() directly — do NOT redirect stdin via </dev/null
         // because xlings may need stdin for subprocess coordination during
         // large package extraction.
-        int directRc = std::system(directCmd.c_str());
-        if constexpr (!mcpp::platform::is_windows) {
-            directRc = WIFEXITED(directRc) ? WEXITSTATUS(directRc) : directRc;
-        }
+        int directRc = mcpp::platform::process::extract_exit_code(
+            std::system(directCmd.c_str()));
         if (directRc == 0) return 0;
     }