Deprecate legacy projects ignoring config settings

In the future the presence of config settings will
auto enable pep517.

Author: sbidoul

news/11915.removal.rst

@@ -0,0 +1,3 @@
+Using ``--config-settings`` with projects that don't have a ``pyproject.toml`` now print
+a deprecation warning. In the future the presence of config settings will automatically
+enable the default build backend for legacy projects and pass the setttings to it.

src/pip/_internal/req/req_install.py

@@ -477,6 +477,15 @@ def load_pyproject_toml(self) -> None:
         )
 
         if pyproject_toml_data is None:
+            if self.config_settings:
+                deprecated(
+                    reason=f"Config settings are ignored for project {self}.",
+                    replacement=(
+                        "to use --use-pep517 or add a "
+                        "pyproject.toml file to the project"
+                    ),
+                    gone_in="23.3",
+                )
             self.use_pep517 = False
             return