Add requirements_to_install to RequirementSet

This property is necessary because the
legacy resolver returns requirements that
need not be installed.

Author: sbidoul

src/pip/_internal/commands/install.py

@@ -356,9 +356,7 @@ def run(self, options: Values, args: List[str]) -> int:
             if options.dry_run:
                 would_install_items = sorted(
                     (r.metadata["name"], r.metadata["version"])
-                    # Use get_installation_order because it does some important
-                    # filtering with the legacy resolver.
-                    for r in resolver.get_installation_order(requirement_set)
+                    for r in requirement_set.requirements_to_install
                 )
                 if would_install_items:
                     write_output(

src/pip/_internal/req/req_set.py

@@ -67,3 +67,16 @@ def get_requirement(self, name: str) -> InstallRequirement:
     @property
     def all_requirements(self) -> List[InstallRequirement]:
         return self.unnamed_requirements + list(self.requirements.values())
+
+    @property
+    def requirements_to_install(self) -> List[InstallRequirement]:
+        """Return the list of requirements that need to be installed.
+
+        TODO remove this property together with the legacy resolver, since the new
+             resolver only returns requirements that need to be installed.
+        """
+        return [
+            install_req
+            for install_req in self.all_requirements
+            if not install_req.constraint and not install_req.satisfied_by
+        ]