Re-organize directory structure (#4971)

See discussion in #2491

Co-authored-by: Ivan Levkivskyi <ilevkivskyi@dropbox.com>

stdlib/posix.pyi

@@ -0,0 +1,165 @@
+import sys
+from builtins import _PathLike  # See comment in builtins
+from os import stat_result as stat_result
+from typing import Dict, List, NamedTuple, Optional, overload
+
+class uname_result(NamedTuple):
+    sysname: str
+    nodename: str
+    release: str
+    version: str
+    machine: str
+
+class times_result(NamedTuple):
+    user: float
+    system: float
+    children_user: float
+    children_system: float
+    elapsed: float
+
+class waitid_result(NamedTuple):
+    si_pid: int
+    si_uid: int
+    si_signo: int
+    si_status: int
+    si_code: int
+
+class sched_param(NamedTuple):
+    sched_priority: int
+
+CLD_CONTINUED: int
+CLD_DUMPED: int
+CLD_EXITED: int
+CLD_TRAPPED: int
+
+EX_CANTCREAT: int
+EX_CONFIG: int
+EX_DATAERR: int
+EX_IOERR: int
+EX_NOHOST: int
+EX_NOINPUT: int
+EX_NOPERM: int
+EX_NOTFOUND: int
+EX_NOUSER: int
+EX_OK: int
+EX_OSERR: int
+EX_OSFILE: int
+EX_PROTOCOL: int
+EX_SOFTWARE: int
+EX_TEMPFAIL: int
+EX_UNAVAILABLE: int
+EX_USAGE: int
+
+F_OK: int
+R_OK: int
+W_OK: int
+X_OK: int
+
+F_LOCK: int
+F_TEST: int
+F_TLOCK: int
+F_ULOCK: int
+
+GRND_NONBLOCK: int
+GRND_RANDOM: int
+NGROUPS_MAX: int
+
+O_APPEND: int
+O_ACCMODE: int
+O_ASYNC: int
+O_CREAT: int
+O_DIRECT: int
+O_DIRECTORY: int
+O_DSYNC: int
+O_EXCL: int
+O_LARGEFILE: int
+O_NDELAY: int
+O_NOATIME: int
+O_NOCTTY: int
+O_NOFOLLOW: int
+O_NONBLOCK: int
+O_RDONLY: int
+O_RDWR: int
+O_RSYNC: int
+O_SYNC: int
+O_TRUNC: int
+O_WRONLY: int
+
+POSIX_FADV_DONTNEED: int
+POSIX_FADV_NOREUSE: int
+POSIX_FADV_NORMAL: int
+POSIX_FADV_RANDOM: int
+POSIX_FADV_SEQUENTIAL: int
+POSIX_FADV_WILLNEED: int
+
+PRIO_PGRP: int
+PRIO_PROCESS: int
+PRIO_USER: int
+
+P_ALL: int
+P_PGID: int
+P_PID: int
+
+RTLD_DEEPBIND: int
+RTLD_GLOBAL: int
+RTLD_LAZY: int
+RTLD_LOCAL: int
+RTLD_NODELETE: int
+RTLD_NOLOAD: int
+RTLD_NOW: int
+
+SCHED_BATCH: int
+SCHED_FIFO: int
+SCHED_IDLE: int
+SCHED_OTHER: int
+SCHED_RESET_ON_FORK: int
+SCHED_RR: int
+
+SEEK_DATA: int
+SEEK_HOLE: int
+
+ST_APPEND: int
+ST_MANDLOCK: int
+ST_NOATIME: int
+ST_NODEV: int
+ST_NODIRATIME: int
+ST_NOEXEC: int
+ST_NOSUID: int
+ST_RDONLY: int
+ST_RELATIME: int
+ST_SYNCHRONOUS: int
+ST_WRITE: int
+
+TMP_MAX: int
+WCONTINUED: int
+
+def WCOREDUMP(__status: int) -> bool: ...
+def WEXITSTATUS(status: int) -> int: ...
+def WIFCONTINUED(status: int) -> bool: ...
+def WIFEXITED(status: int) -> bool: ...
+def WIFSIGNALED(status: int) -> bool: ...
+def WIFSTOPPED(status: int) -> bool: ...
+
+WNOHANG: int
+
+def WSTOPSIG(status: int) -> int: ...
+def WTERMSIG(status: int) -> int: ...
+
+WUNTRACED: int
+
+XATTR_CREATE: int
+XATTR_REPLACE: int
+XATTR_SIZE_MAX: int
+@overload
+def listdir(path: Optional[str] = ...) -> List[str]: ...
+@overload
+def listdir(path: bytes) -> List[bytes]: ...
+@overload
+def listdir(path: int) -> List[str]: ...
+@overload
+def listdir(path: _PathLike[str]) -> List[str]: ...
+
+if sys.platform == "win32":
+    environ: Dict[str, str]
+else:
+    environ: Dict[bytes, bytes]