GH-73991: Add pathlib.Path.copy_into() and move_into() (#123314)

These two methods accept an *existing* directory path, onto which we join
the source path's base name to form the final target path.

A possible alternative implementation is to check for directories in
`copy()` and `move()` and adjust the target path, which is done in several
`shutil` functions. This behaviour is helpful in a shell context, but
less so in a stored program that explicitly specifies destinations. For
example, a user that calls `Path('foo.py').copy('bar.py')` might not
imagine that `bar.py/foo.py` would be created, but under the alternative
implementation this will happen if `bar.py` is an existing directory.

Lib/test/test_pathlib/test_pathlib_abc.py

@@ -2072,6 +2072,20 @@ class DummyPathTest(DummyPurePathTest):
         self.assertTrue(target2.joinpath('link').is_symlink())
         self.assertEqual(target2.joinpath('link').readlink(), self.cls('nonexistent'))
 
+    def test_copy_into(self):
+        base = self.cls(self.base)
+        source = base / 'fileA'
+        target_dir = base / 'dirA'
+        result = source.copy_into(target_dir)
+        self.assertEqual(result, target_dir / 'fileA')
+        self.assertTrue(result.exists())
+        self.assertEqual(source.read_text(), result.read_text())
+
+    def test_copy_into_empty_name(self):
+        source = self.cls('')
+        target_dir = self.base
+        self.assertRaises(ValueError, source.copy_into, target_dir)
+
     def test_move_file(self):
         base = self.cls(self.base)
         source = base / 'fileA'
@@ -2191,6 +2205,22 @@ class DummyPathTest(DummyPurePathTest):
         self.assertTrue(target.is_symlink())
         self.assertEqual(source_readlink, target.readlink())
 
+    def test_move_into(self):
+        base = self.cls(self.base)
+        source = base / 'fileA'
+        source_text = source.read_text()
+        target_dir = base / 'dirA'
+        result = source.move_into(target_dir)
+        self.assertEqual(result, target_dir / 'fileA')
+        self.assertFalse(source.exists())
+        self.assertTrue(result.exists())
+        self.assertEqual(source_text, result.read_text())
+
+    def test_move_into_empty_name(self):
+        source = self.cls('')
+        target_dir = self.base
+        self.assertRaises(ValueError, source.move_into, target_dir)
+
     def test_iterdir(self):
         P = self.cls
         p = P(self.base)