fix(env): ensure all system site paths are used

With this change, when using newer Python versions (>=3.12), Poetry
correctly detects and considers read-only system site packages when
an environment is loaded.

Resolves: python-poetry#9878

src/poetry/utils/env/base_env.py

@@ -61,6 +61,7 @@ def __init__(self, path: Path, base: Path | None = None) -> None:
         self._supported_tags: list[Tag] | None = None
         self._purelib: Path | None = None
         self._platlib: Path | None = None
+        self._fallbacks: list[Path] | None = None
         self._script_dirs: list[Path] | None = None
 
         self._embedded_pip_path: Path | None = None
@@ -175,13 +176,10 @@ def os(self) -> str:
     @property
     def site_packages(self) -> SitePackages:
         if self._site_packages is None:
-            # we disable write checks if no user site exist
-            fallbacks = [self.usersite] if self.usersite else []
             self._site_packages = SitePackages(
                 self.purelib,
                 self.platlib,
-                fallbacks,
-                skip_write_checks=not fallbacks,
+                self.fallbacks,
             )
         return self._site_packages
 
@@ -214,8 +212,16 @@ def platlib(self) -> Path:
 
         return self._platlib
 
+    @property
+    def fallbacks(self) -> list[Path]:
+        if self._fallbacks is None:
+            self._fallbacks = [Path(path) for path in self.paths.get("fallbacks", [])]
+            self._fallbacks += [self.usersite] if self.usersite else []
+
+        return self._fallbacks
+
     def _get_lib_dirs(self) -> list[Path]:
-        return [self.purelib, self.platlib]
+        return [self.purelib, self.platlib, *self.fallbacks]
 
     def is_path_relative_to_lib(self, path: Path) -> bool:
         for lib_path in self._get_lib_dirs():

src/poetry/utils/env/script_strings.py

@@ -97,6 +97,11 @@ def _version_nodot(version):
 
 paths = sysconfig.get_paths().copy()
 
+paths["fallbacks"] = [
+    p for p in site.getsitepackages()
+    if p and p not in {paths.get("purelib"), paths.get("platlib")}
+]
+
 if site.check_enableusersite():
     paths["usersite"] = site.getusersitepackages()
     paths["userbase"] = site.getuserbase()

tests/utils/env/test_env.py

@@ -309,7 +309,7 @@ def test_env_system_packages_are_relative_to_lib(
 
     # These are the virtual environments' base env packages,
     # in this case the system site packages.
-    for dist in metadata.distributions(path=[str(env.parent_env.site_packages.path)]):
+    for dist in env.parent_env.site_packages.distributions():
         assert (
             env.is_path_relative_to_lib(
                 Path(str(dist._path))  # type: ignore[attr-defined]