Do not consider import a.b as b an explicit reexport (#14086)

The point of the `import a as a` and `from a import b as b` syntax for
explicit reexport is that it indicates an intention to do something
different from the ordinary `import a` and `from a import b`.

That is not the case with `import a.b as b`. Even mypy’s own code
includes `import mypy.types as types`, which was not intended to be a
reexport; if it were, it would be written `from mypy import types as
types`.

Pyright agrees that `import a.b as b` should not reexport.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

mypy/semanal.py

@@ -2466,7 +2466,7 @@ def visit_import(self, i: Import) -> None:
             if as_id is not None:
                 base_id = id
                 imported_id = as_id
-                module_public = use_implicit_reexport or id.split(".")[-1] == as_id
+                module_public = use_implicit_reexport or id == as_id
             else:
                 base_id = id.split(".")[0]
                 imported_id = base_id

test-data/unit/check-modules.test

@@ -1818,6 +1818,8 @@ m = n  # E: Cannot assign multiple modules to name "m" without explicit "types.M
 from stub import Iterable  # E: Module "stub" does not explicitly export attribute "Iterable"
 from stub import D  # E: Module "stub" does not explicitly export attribute "D"
 from stub import C
+from stub import foo
+from stub import bar  # E: Module "stub" does not explicitly export attribute "bar"
 
 c = C()
 reveal_type(c.x)  # N: Revealed type is "builtins.int"
@@ -1828,13 +1830,19 @@ reveal_type(it)  # N: Revealed type is "typing.Iterable[builtins.int]"
 from typing import Iterable
 from substub import C as C
 from substub import C as D
+from package import foo as foo
+import package.bar as bar
 
 def fun(x: Iterable[str]) -> Iterable[int]: pass
 
 [file substub.pyi]
 class C:
     x: int
 
+[file package/foo.pyi]
+
+[file package/bar.pyi]
+
 [builtins fixtures/module.pyi]
 
 [case testNoReExportFromStubsMemberType]