install: Raise an error to avoid breaking python-exec

Running pip without --target, --root, or --user will result in packages
being installed systemwide. This has a tendency to break python-exec if
setuptools gets installed or upgraded.

--- pip-19.1/src/pip/_internal/commands/install.py
+++ pip-19.1/src/pip/_internal/commands/install.py
@@ -246,6 +246,9 @@ class InstallCommand(RequirementCommand):
         if options.upgrade:
             upgrade_strategy = options.upgrade_strategy

+        if not options.use_user_site and not options.target_dir and not options.root_path:
+            raise CommandError("(Gentoo) Please run pip with the --user option to avoid breaking python-exec")
+
         if options.build_dir:
             options.build_dir = os.path.abspath(options.build_dir)