fix: fix the module loading of pth files for install cache

Fix #863

news/863.bugfix.md

@@ -0,0 +1 @@
+Fix the module missing error of pywin32 in a virtualenv with `install.cache` set to `true` and caching method is `pth`.

src/pdm/installers/installers.py

@@ -199,7 +199,7 @@ def install_wheel_with_cache(
     package_cache = CachedPackage(cache_path)
     interpreter = str(environment.interpreter.executable)
     script_kind = _get_kind(environment)
-    supports_symlink = (
+    use_symlink = (
         environment.project.config["install.cache_method"] == "symlink"
         and fs_supports_symlink()
     )
@@ -228,17 +228,18 @@ def skip_files(source: WheelFile, element: WheelContentElement) -> bool:
             or path.split("/")[0].endswith(".dist-info")
             # We need to skip the *-nspkg.pth files generated by setuptools'
             # namespace_packages merchanism. See issue#623 for details
-            or not supports_symlink
+            or not use_symlink
             and path.endswith(".pth")
             and not path.endswith("-nspkg.pth")
         )
 
     additional_contents: list[WheelContentElement] = []
     lib_path = package_cache.scheme()["purelib"]
-    if not supports_symlink:
+    if not use_symlink:
         # HACK: Prefix with aaa_ to make it processed as early as possible
         filename = "aaa_" + wheel_stem.split("-")[0] + ".pth"
-        stream = io.BytesIO(f"{lib_path}\n".encode())
+        # use site.addsitedir() rather than a plain path to properly process .pth files
+        stream = io.BytesIO(f"import site;site.addsitedir({lib_path!r})\n".encode())
         additional_contents.append(
             ((filename, "", str(len(stream.getvalue()))), stream, False)
         )
@@ -247,7 +248,7 @@ def skip_files(source: WheelFile, element: WheelContentElement) -> bool:
         scheme_dict=environment.get_paths(),
         interpreter=interpreter,
         script_kind=script_kind,
-        symlink_to=lib_path if supports_symlink else None,
+        symlink_to=lib_path if use_symlink else None,
     )
 
     dist_info_dir = _install_wheel(