[fixup] Fix .egg-link resolution for PEP 660 case

Fixup "Use pip install --editable in easy_install.develop"

Support namespace packages where `spec.submodule_earch_locations` is
a `_NamespacePath` object instead of a simple `list` and add support
for cases where the layout of the source project does not follow the
same structure as the package tree - meaning some custom magic might
be involved in making editable imports work as intended.

src/zc/buildout/easy_install.py

@@ -23,6 +23,7 @@ import distutils.errors
 import distutils.sysconfig
 import errno
 import glob
+import json
 import logging
 import os
 import pkg_resources
@@ -1288,15 +1289,15 @@ def develop(setup, dest, build_ext=None, executable=None, verbosity=-10):
             with open(os.path.join(entry_path, 'top_level.txt')) as f:
                 top_level = f.read().splitlines()
             try:
-                package_paths = subprocess.check_output([
+                package_paths = json.loads(subprocess.check_output([
                     sys.executable, '-S', '-c',
-                    "import importlib.util, site; "
+                    "import importlib.util, json, site; "
                     "site.addsitedir(%(site)r); "
                     "packages = %(packages)r; "
                     "specs = [importlib.util.find_spec(p) for p in packages]; "
-                    "paths = sum((s.submodule_search_locations"
-                    " for s in specs), []); "
-                    "print('\\n'.join(paths))"  % {
+                    "paths = {p: list(s.submodule_search_locations)"
+                    " for p, s in zip(packages, specs)}; "
+                    "print(json.dumps(paths))"  % {
                         'site': lib_dir,
                         'packages': tuple(p for p in top_level if p),
                         }
@@ -1304,20 +1305,47 @@ def develop(setup, dest, build_ext=None, executable=None, verbosity=-10):
                     env=dict(os.environ, PYTHONPATH=''),
                     stderr=subprocess.STDOUT,
                     universal_newlines=True,
-                ).splitlines()
+                ))
             except subprocess.CalledProcessError as e:
                 logger.error(e.output)
                 raise
-            # Resolve import path; warn if there are several and
-            # arbitrarily take the first one.
-            import_paths = [os.path.dirname(p) for p in package_paths]
-            import_path = import_paths[0]
-            if len(set(import_paths)) > 1:
+            # Resolve import path.
+            directory = os.path.normpath(directory)
+            import_paths = []
+            for package, paths in package_paths.items():
+                for path in paths:
+                  # Filter out magic hook values from setuptools PEP 660.
+                  if (path.startswith('__editable__')
+                      and path.endswith('.__path_hook__')): continue
+                  path = os.path.normpath(path)
+                  # Do not parent-path out of the source repository.
+                  if path == directory:
+                      import_paths.append(path)
+                      continue
+                  parent, folder = os.path.split(path)
+                  # Only parent-path if the folder has the name of the package.
+                  import_paths.append(parent if folder == package else path)
+            # Warn if there are several and arbitrarily take the first one.
+            if import_paths:
+                import_path = import_paths[0]
+                unique_import_paths = set(import_paths)
+                if len(unique_import_paths) > 1:
+                    logger.warning(
+                        "Found multiple package import paths"
+                        " in develop project %s\n(\n  %s\n)\n"
+                        "Selected %s" % (
+                          directory,
+                          '\n  '.join(unique_import_paths),
+                          import_path
+                        ))
+            # Warn if no import path is found; fallback to source directory.
+            else:
                 logger.warning(
-                    "Found multiple package import paths in develop project %s"
-                    "\n(\n%s\n)\n"
-                    "Selected %s"
-                    % (directory, '\n'.join(set(import_paths)), import_path))
+                    "Found no package import path in develop project %s "
+                    "for packages %r\n"
+                    "Falling back to project's root directory"
+                    % (directory, tuple(packages)),)
+                import_path = directory
             # Mark this dist as installed by buildout as develop
             if entry.endswith('.dist-info'):
                 with open(os.path.join(entry_path, 'INSTALLER'), 'w') as f: