Support forward slash on windows (#4563)

Support forward slash on windows

news/4563.feature

@@ -0,0 +1 @@
+Local Packages can now be referenced using forward slashes on Windows.

pip/req/req_install.py

@@ -194,9 +194,12 @@ class InstallRequirement(object):
             link = Link(name)
         else:
             p, extras = _strip_extras(path)
-            if (os.path.isdir(p) and
-                    (os.path.sep in name or name.startswith('.'))):
-
+            looks_like_dir = os.path.isdir(p) and (
+                os.path.sep in name or
+                (os.path.altsep is not None and os.path.altsep in name) or
+                name.startswith('.')
+            )
+            if looks_like_dir:
                 if not is_installable_dir(p):
                     raise InstallationError(
                         "Directory %r is not installable. File 'setup.py' "

tests/unit/test_req_install.py

@@ -26,3 +26,18 @@ class TestInstallRequirementBuildDirectory(object):
             assert os.path.dirname(tmp_build_dir) == os.path.dirname(tmp_dir)
         os.rmdir(tmp_dir)
         assert not os.path.exists(tmp_dir)
+
+    def test_forward_slash_results_in_a_link(self, tmpdir):
+        install_dir = tmpdir / "foo" / "bar"
+
+        # Just create a file for letting the logic work
+        setup_py_path = install_dir / "setup.py"
+        os.makedirs(str(install_dir))
+        with open(setup_py_path, 'w') as f:
+            f.write('')
+
+        requirement = InstallRequirement.from_line(
+            str(install_dir).replace(os.sep, os.altsep or os.sep)
+        )
+
+        assert requirement.link is not None