avoid importing sysconfig during startup

site.py requires only a couple of functions from sysconfig, but needs to
load the entirety of sysconfig to get those functions. This commit
makes it such that sysconfig is imported only when sys.platform is darwin.

third_party/python/Lib/site.py

@@ -234,6 +234,35 @@ def check_enableusersite():
 
     return True
 
+def _getuserbase():
+    env_base = os.environ.get("PYTHONUSERBASE", None)
+
+    def joinuser(*args):
+        return os.path.expanduser(os.path.join(*args))
+
+    if os.name == "nt":
+        base = os.environ.get("APPDATA") or "~"
+        if env_base:
+            return env_base
+        else:
+            return joinuser(base, "Python")
+
+    if sys.platform == "darwin":
+        from sysconfig import get_config_var
+        framework = get_config_var("PYTHONFRAMEWORK")
+        if framework:
+            if env_base:
+                return env_base
+            else:
+                return joinuser("~", "Library", framework, "%d.%d" %
+                                sys.version_info[:2])
+
+    if env_base:
+        return env_base
+    else:
+        return joinuser("~", ".local")
+
+
 def getuserbase():
     """Returns the `user base` directory path.
 
@@ -244,8 +273,7 @@ def getuserbase():
     global USER_BASE
     if USER_BASE is not None:
         return USER_BASE
-    from sysconfig import get_config_var
-    USER_BASE = get_config_var('userbase')
+    USER_BASE = _getuserbase()
     return USER_BASE
 
 def getusersitepackages():
@@ -255,20 +283,22 @@ def getusersitepackages():
     function will also set it.
     """
     global USER_SITE
-    user_base = getuserbase() # this will also set USER_BASE
 
     if USER_SITE is not None:
         return USER_SITE
 
-    from sysconfig import get_path
-
     if sys.platform == 'darwin':
-        from sysconfig import get_config_var
+        from sysconfig import get_config_var, get_path
         if get_config_var('PYTHONFRAMEWORK'):
             USER_SITE = get_path('purelib', 'osx_framework_user')
             return USER_SITE
 
-    USER_SITE = get_path('purelib', '%s_user' % os.name)
+    user_base = getuserbase() # this will also set USER_BASE
+    purelib_map = {
+        "posix_user":'{userbase}/lib/python3.6/site-packages',
+        "nt_user": "{userbase}/Python36/site-packages",
+    }
+    USER_SITE = purelib_map.get('%s_user' % os.name).format(userbase=user_base)
     return USER_SITE
 
 def addusersitepackages(known_paths):