Better detection of invalid __legacy__ PEP 517 projects

This errors out on PEP 517 projects using the
`setuptools.build_meta:__legacy__` backend but don't have a `setup.py`.

This now errors out on:
- `pip install file://$PWD` of an empty directory
- `pip install file://$PWD` of directory with a pyproject.toml but no
- `pip install file://$PWD` of directory with a pyproject.toml but no
build-system table and no setup.py

Author: sbidoul

src/pip/_internal/pyproject.py

@@ -174,4 +174,12 @@ def load_pyproject_toml(
         backend = "setuptools.build_meta:__legacy__"
         check = ["setuptools>=40.8.0", "wheel"]
 
+    if (
+        build_system["build-backend"] == "setuptools.build_meta:__legacy__"
+        and not has_setup
+    ):
+        raise InstallationError(
+            f"File 'setup.py' not found for legacy project {req_name}."
+        )
+
     return BuildSystemDetails(requires, backend, check, backend_path)

src/pip/_internal/req/constructors.py

@@ -25,7 +25,6 @@
 from pip._internal.req.req_file import ParsedRequirement
 from pip._internal.req.req_install import InstallRequirement
 from pip._internal.utils.filetypes import is_archive_file
-from pip._internal.utils.misc import is_installable_dir
 from pip._internal.utils.urls import path_to_url
 from pip._internal.vcs import is_url, vcs
 
@@ -232,12 +231,7 @@ def _get_url_from_path(path: str, name: str) -> Optional[str]:
     an @, it will treat it as a PEP 440 URL requirement and return the path.
     """
     if _looks_like_path(name) and os.path.isdir(path):
-        if is_installable_dir(path):
-            return path_to_url(path)
-        raise InstallationError(
-            f"Directory {name!r} is not installable. Neither 'setup.py' "
-            "nor 'pyproject.toml' found."
-        )
+        return path_to_url(path)
     if not is_archive_file(path):
         return None
     if os.path.isfile(path):