Cache find_best_candidate results

This is possible because self.make_candidate_evaluator only depends
on:
- the function arguments which are keys to the cache
- self._target_python which never changes during a pip resolution
- self._candidate_prefs which never changes during a pip resolution

On a fresh install, pip install <a package with ~ 100 dependencies>
runs on my machine in:

master (a0e34e9cf7)
=======================

0m33.058s
0m34.105s
0m32.426s

This commit
===========

0m15.860s
0m16.254s
0m15.910s

pip 20.2.4 - legacy resolver
============================

0m15.145s
0m15.040s
0m15.152s

src/pip/_internal/index/package_finder.py

@@ -863,6 +863,7 @@ class PackageFinder(object):
             hashes=hashes,
         )
 
+    @lru_cache(maxsize=None)
     def find_best_candidate(
         self,
         project_name,       # type: str