Re-organize directory structure (#4971)

See discussion in #2491

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

stdlib/unittest/loader.pyi

@@ -0,0 +1,50 @@
+import sys
+import unittest.case
+import unittest.result
+import unittest.suite
+from types import ModuleType
+from typing import Any, Callable, List, Optional, Sequence, Type
+
+_SortComparisonMethod = Callable[[str, str], int]
+_SuiteClass = Callable[[List[unittest.case.TestCase]], unittest.suite.TestSuite]
+
+class TestLoader:
+    errors: List[Type[BaseException]]
+    testMethodPrefix: str
+    sortTestMethodsUsing: _SortComparisonMethod
+
+    if sys.version_info >= (3, 7):
+        testNamePatterns: Optional[List[str]]
+
+    suiteClass: _SuiteClass
+    def loadTestsFromTestCase(self, testCaseClass: Type[unittest.case.TestCase]) -> unittest.suite.TestSuite: ...
+    def loadTestsFromModule(self, module: ModuleType, *args: Any, pattern: Any = ...) -> unittest.suite.TestSuite: ...
+    def loadTestsFromName(self, name: str, module: Optional[ModuleType] = ...) -> unittest.suite.TestSuite: ...
+    def loadTestsFromNames(self, names: Sequence[str], module: Optional[ModuleType] = ...) -> unittest.suite.TestSuite: ...
+    def getTestCaseNames(self, testCaseClass: Type[unittest.case.TestCase]) -> Sequence[str]: ...
+    def discover(self, start_dir: str, pattern: str = ..., top_level_dir: Optional[str] = ...) -> unittest.suite.TestSuite: ...
+
+defaultTestLoader: TestLoader
+
+if sys.version_info >= (3, 7):
+    def getTestCaseNames(
+        testCaseClass: Type[unittest.case.TestCase],
+        prefix: str,
+        sortUsing: _SortComparisonMethod = ...,
+        testNamePatterns: Optional[List[str]] = ...,
+    ) -> Sequence[str]: ...
+
+else:
+    def getTestCaseNames(
+        testCaseClass: Type[unittest.case.TestCase], prefix: str, sortUsing: _SortComparisonMethod = ...
+    ) -> Sequence[str]: ...
+
+def makeSuite(
+    testCaseClass: Type[unittest.case.TestCase],
+    prefix: str = ...,
+    sortUsing: _SortComparisonMethod = ...,
+    suiteClass: _SuiteClass = ...,
+) -> unittest.suite.TestSuite: ...
+def findTestCases(
+    module: ModuleType, prefix: str = ..., sortUsing: _SortComparisonMethod = ..., suiteClass: _SuiteClass = ...
+) -> unittest.suite.TestSuite: ...