bpo-45413: Define "posix_venv", "nt_venv" and "venv" sysconfig installation schemes (GH-31034)

Define *posix_venv* and *nt_venv* sysconfig installation schemes
to be used for bootstrapping new virtual environments.
Add *venv* sysconfig installation scheme to get the appropriate one of the above.
The schemes are identical to the pre-existing
*posix_prefix* and *nt* install schemes.
The venv module now uses the *venv* scheme to create new virtual environments
instead of hardcoding the paths depending only on the platform. Downstream
Python distributors customizing the *posix_prefix* or *nt* install
scheme in a way that is not compatible with the install scheme used in
virtual environments are encouraged not to customize the *venv* schemes.
When Python itself runs in a virtual environment,
sysconfig.get_default_scheme and
sysconfig.get_preferred_scheme with `key="prefix"` returns
*venv*.

Lib/sysconfig.py

@@ -56,8 +56,53 @@ _INSTALL_SCHEMES = {
         'scripts': '{base}/Scripts',
         'data': '{base}',
         },
+    # Downstream distributors can overwrite the default install scheme.
+    # This is done to support downstream modifications where distributors change
+    # the installation layout (eg. different site-packages directory).
+    # So, distributors will change the default scheme to one that correctly
+    # represents their layout.
+    # This presents an issue for projects/people that need to bootstrap virtual
+    # environments, like virtualenv. As distributors might now be customizing
+    # the default install scheme, there is no guarantee that the information
+    # returned by sysconfig.get_default_scheme/get_paths is correct for
+    # a virtual environment, the only guarantee we have is that it is correct
+    # for the *current* environment. When bootstrapping a virtual environment,
+    # we need to know its layout, so that we can place the files in the
+    # correct locations.
+    # The "*_venv" install scheme is a scheme to bootstrap virtual environments,
+    # essentially identical to the default posix_prefix/nt schemes.
+    # Downstream distributors who patch posix_prefix/nt scheme are encouraged to
+    # leave the following schemes unchanged
+    'posix_venv': {
+        'stdlib': '{installed_base}/{platlibdir}/python{py_version_short}',
+        'platstdlib': '{platbase}/{platlibdir}/python{py_version_short}',
+        'purelib': '{base}/lib/python{py_version_short}/site-packages',
+        'platlib': '{platbase}/{platlibdir}/python{py_version_short}/site-packages',
+        'include':
+            '{installed_base}/include/python{py_version_short}{abiflags}',
+        'platinclude':
+            '{installed_platbase}/include/python{py_version_short}{abiflags}',
+        'scripts': '{base}/bin',
+        'data': '{base}',
+        },
+    'nt_venv': {
+        'stdlib': '{installed_base}/Lib',
+        'platstdlib': '{base}/Lib',
+        'purelib': '{base}/Lib/site-packages',
+        'platlib': '{base}/Lib/site-packages',
+        'include': '{installed_base}/Include',
+        'platinclude': '{installed_base}/Include',
+        'scripts': '{base}/Scripts',
+        'data': '{base}',
+        },
     }
 
+# For the OS-native venv scheme, we essentially provide an alias:
+if os.name == 'nt':
+    _INSTALL_SCHEMES['venv'] = _INSTALL_SCHEMES['nt_venv']
+else:
+    _INSTALL_SCHEMES['venv'] = _INSTALL_SCHEMES['posix_venv']
+
 
 # NOTE: site.py has copy of this function.
 # Sync it when modify this function.
@@ -251,6 +296,8 @@ def _get_preferred_schemes():
 
 
 def get_preferred_scheme(key):
+    if key == 'prefix' and sys.prefix != sys.base_prefix:
+        return 'venv'
     scheme = _get_preferred_schemes()[key]
     if scheme not in _INSTALL_SCHEMES:
         raise ValueError(