Add requested property to BaseDistribution

Author: sbidoul

src/pip/_internal/metadata/base.py

@@ -311,6 +311,10 @@ def installer(self) -> str:
                 return cleaned_line
         return ""
 
+    @property
+    def requested(self) -> bool:
+        return self.is_file("REQUESTED")
+
     @property
     def editable(self) -> bool:
         return bool(self.editable_project_location)