Instead of detecting 'get-pip' during the import of pip, detect the a…

…ttempt to 'import setuptools' during 'get-pip', and in that case, stub the import to signal the presence of setuptools. Ref #3022. Fixes #2993.

_distutils_hack/__init__.py

@@ -136,11 +136,36 @@ def spec_for_pip(self):
         """
         if self.pip_imported_during_build():
             return
-        if self.is_get_pip():
-            return
         clear_distutils()
         self.spec_for_distutils = lambda: None
 
+    def spec_for_setuptools(self):
+        """
+        get-pip imports setuptools solely for the purpose of
+        determining if it's installed. In this case, provide
+        a stubbed spec to represent setuptools being present
+        without invoking any behavior.
+
+        Workaround for pypa/get-pip#137.
+        """
+        if not self.is_get_pip():
+            return
+
+        import importlib
+
+        class StubbedLoader(importlib.abc.Loader):
+
+            def create_module(self, spec):
+                import types
+                return types.ModuleType('setuptools')
+
+            def exec_module(self, module):
+                pass
+
+        return importlib.util.spec_from_loader(
+            'setuptools', StubbedLoader(),
+        )
+
     @classmethod
     def pip_imported_during_build(cls):
         """