Have a nice error when setuptools is not available

pip/req.py

@@ -261,6 +261,15 @@ class InstallRequirement(object):
 
     @property
     def setup_py(self):
+        try:
+            import pkg_resources
+        except ImportError:
+            # Setuptools is not available
+            raise InstallationError(
+                "setuptools must be installed to install from a source "
+                "distribution"
+            )
+
         setup_file = 'setup.py'
 
         if self.editable_options and 'subdirectory' in self.editable_options:

tests/functional/test_install.py

@@ -13,6 +13,18 @@ from tests.lib.local_repos import local_checkout
 from tests.lib.path import Path
 
 
+def test_without_setuptools(script):
+    script.run("pip", "uninstall", "setuptools", "-y")
+    result = script.run(
+        "python", "-m", "pip", "install", "INITools==0.2", "--no-use-wheel",
+        expect_error=True,
+    )
+    assert (
+        "setuptools must be installed to install from a source distribution"
+            in result.stdout
+    )
+
+
 def test_pip_second_command_line_interface_works(script):
     """
     Check if ``pip<PYVERSION>`` commands behaves equally