Fix skip site-packages for web packaging (#199)

* Unify site-packages handling and copy for Pyodide

Compute a single sitePackagesRoot (defaulting to build/site-packages but overridden by the environment variable) before installing requirements. Use that path when installing requirements, and for the Pyodide platform copy the site-packages into the temp app archive directory (defaultSitePackagesDir). Also cleans up nullable logic and minor variable handling.

* Copy site-packages for Pyodide only when needed

Add a guard to only copy site-packages for the Pyodide platform if there are requirements to include (requirements.isNotEmpty). This prevents unnecessary copying of the site-packages directory when no requirements are specified, avoiding wasted I/O and archive bloat.

Author: FeodorFitsner

src/serious_python/bin/package_command.dart

@@ -260,23 +260,20 @@ class PackageCommand extends Command {
         await cleanupDir(tempDir, allJunkFiles);
       }
 
-      // install requirements
-      if (requirements.isNotEmpty && !skipSitePackages) {
-        String? sitePackagesRoot;
-
-        if (platform != "Pyodide") {
-          if (Platform.environment
-              .containsKey(sitePackagesEnvironmentVariable)) {
-            sitePackagesRoot =
-                Platform.environment[sitePackagesEnvironmentVariable];
-          }
-          if (sitePackagesRoot == null || sitePackagesRoot.isEmpty) {
-            sitePackagesRoot = path.join(currentPath, "build", "site-packages");
-          }
-        } else {
-          sitePackagesRoot = path.join(tempDir.path, defaultSitePackagesDir);
+      // site-packages root
+      String sitePackagesRoot =
+          path.join(currentPath, "build", "site-packages");
+      if (Platform.environment
+          .containsKey(sitePackagesEnvironmentVariable)) {
+        final envValue =
+            Platform.environment[sitePackagesEnvironmentVariable];
+        if (envValue != null && envValue.isNotEmpty) {
+          sitePackagesRoot = envValue;
         }
+      }
 
+      // install requirements
+      if (requirements.isNotEmpty && !skipSitePackages) {
         if (await Directory(sitePackagesRoot).exists()) {
           await for (var f in Directory(sitePackagesRoot)
               .list()
@@ -432,6 +429,21 @@ class PackageCommand extends Command {
         }
       }
 
+      // copy site packages to temp dir for web platform
+      if (platform == "Pyodide" && requirements.isNotEmpty) {
+        final sitePackagesSrcDir = Directory(sitePackagesRoot);
+        if (await sitePackagesSrcDir.exists()) {
+          stdout.writeln("Copying site packages to app archive");
+          final webPkgDir =
+              Directory(path.join(tempDir.path, defaultSitePackagesDir));
+          if (!await webPkgDir.exists()) {
+            await webPkgDir.create(recursive: true);
+          }
+          await copyDirectory(
+              sitePackagesSrcDir, webPkgDir, sitePackagesSrcDir.path, []);
+        }
+      }
+
       // create archive
       stdout.writeln(
           "Creating app archive at ${dest.path} from a temp directory");