Don't reexport star-imported symbols when implicit reexports are disabled

Currently, in regular files, symbols imported into module A with `from B
import *` are considered explicitly exported from module B as well, i.e.
they are considered part of module's B public API.

This behavior makes sense in stub files as a shorthand (and is specified
in PEP 484), but for regular files I think it is better to be explicit:
add the symbols to `__all__`.

Further discussion:
python#7042 (comment)

Author: bluetech

mypy/semanal.py

@@ -1869,7 +1869,15 @@ def visit_import_all(self, i: ImportAll) -> None:
                         if self.process_import_over_existing_name(
                                 name, existing_symbol, node, i):
                             continue
-                    self.add_imported_symbol(name, node, i)
+                    # In stub files, `from x import *` always reexports the symbols.
+                    # In regular files, only if implicit reexports are enabled.
+                    module_public = (
+                        self.is_stub_file
+                        or self.options.implicit_reexport
+                    )
+                    self.add_imported_symbol(name, node, i,
+                                             module_public=module_public,
+                                             module_hidden=not module_public)
         else:
             # Don't add any dummy symbols for 'from x import *' if 'x' is unknown.
             pass

test-data/unit/check-flags.test

@@ -1158,6 +1158,29 @@ __all__ = ('b',)
 [out]
 main:2: error: Module 'other_module_2' has no attribute 'a'
 
+[case testNoImplicitReexportStarConsideredImplicit]
+# flags: --no-implicit-reexport
+from other_module_2 import a
+[file other_module_1.py]
+a = 5
+[file other_module_2.py]
+from other_module_1 import *
+[out]
+main:2: error: Module 'other_module_2' has no attribute 'a'
+
+[case testNoImplicitReexportStarCanBeReexportedWithAll]
+# flags: --no-implicit-reexport
+from other_module_2 import a
+from other_module_2 import b
+[file other_module_1.py]
+a = 5
+b = 6
+[file other_module_2.py]
+from other_module_1 import *
+__all__ = ('b',)
+[out]
+main:2: error: Module 'other_module_2' has no attribute 'a'
+
 [case testNoImplicitReexportMypyIni]
 # flags: --config-file tmp/mypy.ini
 from other_module_2 import a