Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/encodings/__init__.pyi

@@ -0,0 +1,7 @@
+import codecs
+from typing import Any
+
+def search_function(encoding: str) -> codecs.CodecInfo: ...
+
+# Explicitly mark this package as incomplete.
+def __getattr__(name: str) -> Any: ...

stdlib/encodings/utf_8.pyi

@@ -0,0 +1,15 @@
+import codecs
+from typing import Tuple
+
+class IncrementalEncoder(codecs.IncrementalEncoder):
+    def encode(self, input: str, final: bool = ...) -> bytes: ...
+
+class IncrementalDecoder(codecs.BufferedIncrementalDecoder):
+    def _buffer_decode(self, input: bytes, errors: str, final: bool) -> Tuple[str, int]: ...
+
+class StreamWriter(codecs.StreamWriter): ...
+class StreamReader(codecs.StreamReader): ...
+
+def getregentry() -> codecs.CodecInfo: ...
+def encode(input: str, errors: str = ...) -> bytes: ...
+def decode(input: bytes, errors: str = ...) -> str: ...