Add docstring to emphasise laziness

Author: uranusjr

src/pip/_internal/resolution/resolvelib/found_candidates.py

@@ -1,3 +1,13 @@
+"""Utilities to lazily create and visit candidates found.
+
+Creating and visiting a candidate is a *very* costly operation. It involves
+fetching, extracting, potentially building modules from source, and verifying
+distribution metadata. It is therefore crucial for performance to keep
+everything here lazy all the way down, so we only touch candidates that we
+absolutely need, and not "download the world" when we only need one version of
+something.
+"""
+
 import itertools
 
 from pip._vendor.six.moves import collections_abc  # type: ignore