Split stdlib into Python 2 and 3 versions (#5442)

All new files in stdlib/@python2 are straight copies of the
corresponding files in stdlib.

stdlib/@python2/modulefinder.pyi

@@ -0,0 +1,76 @@
+import sys
+from types import CodeType
+from typing import IO, Any, Container, Dict, Iterable, Iterator, List, Optional, Sequence, Tuple
+
+LOAD_CONST: int  # undocumented
+IMPORT_NAME: int  # undocumented
+STORE_NAME: int  # undocumented
+STORE_GLOBAL: int  # undocumented
+STORE_OPS: Tuple[int, int]  # undocumented
+EXTENDED_ARG: int  # undocumented
+
+packagePathMap: Dict[str, List[str]]  # undocumented
+
+def AddPackagePath(packagename: str, path: str) -> None: ...
+
+replacePackageMap: Dict[str, str]  # undocumented
+
+def ReplacePackage(oldname: str, newname: str) -> None: ...
+
+class Module:  # undocumented
+    def __init__(self, name: str, file: Optional[str] = ..., path: Optional[str] = ...) -> None: ...
+    def __repr__(self) -> str: ...
+
+class ModuleFinder:
+
+    modules: Dict[str, Module]
+    path: List[str]  # undocumented
+    badmodules: Dict[str, Dict[str, int]]  # undocumented
+    debug: int  # undocumented
+    indent: int  # undocumented
+    excludes: Container[str]  # undocumented
+    replace_paths: Sequence[Tuple[str, str]]  # undocumented
+
+    if sys.version_info >= (3, 8):
+        def __init__(
+            self,
+            path: Optional[List[str]] = ...,
+            debug: int = ...,
+            excludes: Optional[Container[str]] = ...,
+            replace_paths: Optional[Sequence[Tuple[str, str]]] = ...,
+        ) -> None: ...
+    else:
+        def __init__(
+            self,
+            path: Optional[List[str]] = ...,
+            debug: int = ...,
+            excludes: Container[str] = ...,
+            replace_paths: Sequence[Tuple[str, str]] = ...,
+        ) -> None: ...
+    def msg(self, level: int, str: str, *args: Any) -> None: ...  # undocumented
+    def msgin(self, *args: Any) -> None: ...  # undocumented
+    def msgout(self, *args: Any) -> None: ...  # undocumented
+    def run_script(self, pathname: str) -> None: ...
+    def load_file(self, pathname: str) -> None: ...  # undocumented
+    def import_hook(
+        self, name: str, caller: Optional[Module] = ..., fromlist: Optional[List[str]] = ..., level: int = ...
+    ) -> Optional[Module]: ...  # undocumented
+    def determine_parent(self, caller: Optional[Module], level: int = ...) -> Optional[Module]: ...  # undocumented
+    def find_head_package(self, parent: Module, name: str) -> Tuple[Module, str]: ...  # undocumented
+    def load_tail(self, q: Module, tail: str) -> Module: ...  # undocumented
+    def ensure_fromlist(self, m: Module, fromlist: Iterable[str], recursive: int = ...) -> None: ...  # undocumented
+    def find_all_submodules(self, m: Module) -> Iterable[str]: ...  # undocumented
+    def import_module(self, partname: str, fqname: str, parent: Module) -> Optional[Module]: ...  # undocumented
+    def load_module(self, fqname: str, fp: IO[str], pathname: str, file_info: Tuple[str, str, str]) -> Module: ...  # undocumented
+    if sys.version_info >= (3, 6):
+        def scan_opcodes(self, co: CodeType) -> Iterator[Tuple[str, Tuple[Any, ...]]]: ...  # undocumented
+    def scan_code(self, co: CodeType, m: Module) -> None: ...  # undocumented
+    def load_package(self, fqname: str, pathname: str) -> Module: ...  # undocumented
+    def add_module(self, fqname: str) -> Module: ...  # undocumented
+    def find_module(
+        self, name: str, path: Optional[str], parent: Optional[Module] = ...
+    ) -> Tuple[Optional[IO[Any]], Optional[str], Tuple[str, str, int]]: ...  # undocumented
+    def report(self) -> None: ...
+    def any_missing(self) -> List[str]: ...  # undocumented
+    def any_missing_maybe(self) -> Tuple[List[str], List[str]]: ...  # undocumented
+    def replace_paths_in_code(self, co: CodeType) -> CodeType: ...  # undocumented