Make virtualenv use sysconfig to find paths

Instead of having our own copies of what we believe paths should be,
or manually mutating what sysconfig returns for other variables, we
should just delegate all of this to sysconfig in the standard way.

This fixes #49545, fixing Python 3.12.8 support, where sysconfig
stopped overly caching certain variables, thus leading to oddities
after we've rewritten sys.exec_prefix and sys.prefix.

tools/wpt/virtualenv.py

@@ -6,7 +6,6 @@
 import site
 import sys
 import sysconfig
-from pathlib import Path
 from shutil import which
 
 # The `pkg_resources` module is provided by `setuptools`, which is itself a
@@ -50,11 +49,40 @@ def create(self):
             self._working_set = None
         call(*self.virtualenv, self.path)
 
+    def get_paths(self):
+        """Wrapper around sysconfig.get_paths(), returning the appropriate paths for the env."""
+        if "venv" in sysconfig.get_scheme_names():
+            # This should always be used on Python 3.11 and above.
+            scheme = "venv"
+        elif os.name == "nt":
+            # This matches nt_venv, unless sysconfig has been modified.
+            scheme = "nt"
+        elif os.name == "posix":
+            # This matches posix_venv, unless sysconfig has been modified.
+            scheme = "posix_prefix"
+        elif sys.version_info >= (3, 10):
+            # Using the default scheme is somewhat fragile, as various Python
+            # distributors (e.g., what Debian and Fedora package, and what Xcode
+            # includes) change the default scheme away from the upstream
+            # defaults, but it's about as good as we can do.
+            scheme = sysconfig.get_default_scheme()
+        else:
+            # This is explicitly documented as having previously existed in the 3.10
+            # docs, and has existed since CPython 2.7 and 3.1 (but not 3.0).
+            scheme = sysconfig._get_default_scheme()
+
+        vars = {
+            "base": self.path,
+            "platbase": self.path,
+            "installed_base": self.path,
+            "installed_platbase": self.path,
+        }
+
+        return sysconfig.get_paths(scheme, vars)
+
     @property
     def bin_path(self):
-        if sys.platform in ("win32", "cygwin"):
-            return os.path.join(self.path, "Scripts")
-        return os.path.join(self.path, "bin")
+        return self.get_paths()["scripts"]
 
     @property
     def pip_path(self):
@@ -67,24 +95,10 @@ def pip_path(self):
 
     @property
     def lib_path(self):
-        base = self.path
-
-        # this block is literally taken from virtualenv 16.4.3
-        IS_PYPY = hasattr(sys, "pypy_version_info")
-        IS_JYTHON = sys.platform.startswith("java")
-        if IS_JYTHON:
-            site_packages = os.path.join(base, "Lib", "site-packages")
-        elif IS_PYPY:
-            site_packages = os.path.join(base, "site-packages")
-        else:
-            IS_WIN = sys.platform == "win32"
-            if IS_WIN:
-                site_packages = os.path.join(base, "Lib", "site-packages")
-            else:
-                version = f"{sys.version_info.major}.{sys.version_info.minor}"
-                site_packages = os.path.join(base, "lib", f"python{version}", "site-packages")
-
-        return site_packages
+        # We always return platlib here, even if it differs to purelib, because we can
+        # always install pure-Python code into the platlib safely too. It's also very
+        # unlikely to differ for a venv.
+        return self.get_paths()["platlib"]
 
     @property
     def working_set(self):
@@ -97,43 +111,35 @@ def working_set(self):
         return self._working_set
 
     def activate(self):
-        if sys.platform == 'darwin':
+        if sys.platform == "darwin":
             # The default Python on macOS sets a __PYVENV_LAUNCHER__ environment
             # variable which affects invocation of python (e.g. via pip) in a
             # virtualenv. Unset it if present to avoid this. More background:
             # https://github.com/web-platform-tests/wpt/issues/27377
             # https://github.com/python/cpython/pull/9516
-            os.environ.pop('__PYVENV_LAUNCHER__', None)
+            os.environ.pop("__PYVENV_LAUNCHER__", None)
+
+        paths = self.get_paths()
 
         # Setup the path and site packages as if we'd launched with the virtualenv active
-        bin_dir = os.path.join(self.path, "bin")
+        bin_dir = paths["scripts"]
         os.environ["PATH"] = os.pathsep.join([bin_dir] + os.environ.get("PATH", "").split(os.pathsep))
+
+        # While not required (`./venv/bin/python3` won't set it, but
+        # `source ./venv/bin/activate && python3` will), we have historically set this.
         os.environ["VIRTUAL_ENV"] = self.path
 
         prev_length = len(sys.path)
 
-        schemes = sysconfig.get_scheme_names()
-        if "venv" in schemes:
-            scheme = "venv"
-        else:
-            scheme = "nt" if os.name == "nt" else "posix_user"
-        sys_paths = sysconfig.get_paths(scheme)
-        data_path = sys_paths["data"]
-        added = set()
         # Add the venv library paths as sitedirs.
-        # This converts system paths like /usr/local/lib/python3.10/site-packages
-        # to venv-relative paths like {self.path}/lib/python3.10/site-packages and adds
-        # those paths as site dirs to be used for module import.
         for key in ["purelib", "platlib"]:
-            host_path = Path(sys_paths[key])
-            relative_path = host_path.relative_to(data_path)
-            site_dir = os.path.normpath(os.path.normcase(Path(self.path) / relative_path))
-            if site_dir not in added:
-                site.addsitedir(site_dir)
-                added.add(site_dir)
+            site.addsitedir(paths[key])
+
+        # Rearrange the path
         sys.path[:] = sys.path[prev_length:] + sys.path[0:prev_length]
 
-        sys.real_prefix = sys.prefix
+        # Change prefixes, similar to what initconfig/site does for venvs.
+        sys.exec_prefix = self.path
         sys.prefix = self.path
 
     def start(self):