List out helpful info on basic resolution failures

If there was a failure in resolution of a requirement that stemmed from
a simple fact of there being no candidate found that could possibly
satisfy it, make pip output the following diagnostic information as a
warning:
- Which versions of the package are available to pip (outputs "(none)"
if there weren't any).
- For the above versions, in which indexes were they found (outputs
"(none)" if there weren't any)
- Wheel cache directory path (outputs "(not used)" if cache is not
used).

Author: MrMino

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

@@ -430,6 +430,22 @@ def get_installation_error(self, e):
                 "Could not find a version that satisfies the requirement %s",
                 req_disp,
             )
+
+            acands = self._finder.find_all_candidates(req.project_name)
+            aversions = [str(cand.version) for cand in acands]
+            afrom = set(str(cand.link.comes_from) for cand in acands)
+            cache_dir = (self._wheel_cache.cache_dir
+                         if self._wheel_cache is not None else None)
+            diagnostic_hint = (
+                f"\nAvailable versions for '{req.project_name}':\n"
+                f"{', '.join(aversions) if aversions else '(none)'}\n\n"
+                f"Found in index{'es' if len(afrom) > 1 else ''}:\n"
+                f"{', '.join(afrom) if afrom else '(none)'}\n\n"
+                f"Cache dir:\n"
+                f"{cache_dir or '(not used)'}"
+            )
+            logger.warning(diagnostic_hint)
+
             return DistributionNotFound(
                 f'No matching distribution found for {req}'
             )