Restore expectation that egg-link files will be named with dashes and…

… not underscores for compatibility with older pips.

setuptools/_normalization.py

@@ -120,6 +120,21 @@ def filename_component(value: str) -> str:
     return value.replace("-", "_").strip("_")
 
 
+def filename_component_broken(value: str) -> str:
+    """
+    Produce the incorrect filename component for compatibility.
+
+    See pypa/setuptools#4167 for detailed analysis.
+
+    TODO: replace this with filename_component after pip 24 is
+    nearly-ubiquitous.
+
+    >>> filename_component_broken('foo_bar-baz')
+    'foo-bar-baz'
+    """
+    return value.replace('_', '-')
+
+
 def safer_name(value: str) -> str:
     """Like ``safe_name`` but can be used as filename component for wheel"""
     # See bdist_wheel.safer_name

setuptools/command/develop.py

@@ -5,6 +5,7 @@
 import glob
 
 from setuptools.command.easy_install import easy_install
+from setuptools import _normalization
 from setuptools import _path
 from setuptools import namespaces
 import setuptools
@@ -52,7 +53,9 @@ def finalize_options(self):
         # pick up setup-dir .egg files only: no .egg-info
         self.package_index.scan(glob.glob('*.egg'))
 
-        egg_link_fn = ei.egg_name + '.egg-link'
+        egg_link_fn = (
+            _normalization.filename_component_broken(ei.egg_name) + '.egg-link'
+        )
         self.egg_link = os.path.join(self.install_dir, egg_link_fn)
         self.egg_base = ei.egg_base
         if self.egg_path is None: