Minimize Python startup imports (#292)

* get_exports_list should return list
* remove unintentional `CC=clang` in makefile
* 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.

* remove redundant constants from stat module

The constants are only there in case the C implementation (ie the _stat
module) is not available. With Cosmopolitan the _stat module is always
available. The entire Lib/stat.py file can be removed if the Windows-based
constants can be moved into the Modules/_stat.c.

* minimal changes to os.py

python checks os-based assumptions at startup, some of  which can be
bypassed since this is Cosmopolitan Python.

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):