Display dependency chain on each Collecting line

This tremendously helps understand why a package is being fetched and
can help investigate and fix dependency resolver backtracking issues
when incoherent constraints/package sets are provided or when new
versions of a package trigger a completely different backtracking
strategy, leading to very hard to debug situations.

src/pip/_internal/operations/prepare.py

@@ -268,6 +268,15 @@ def _log_preparing_link(self, req: InstallRequirement) -> None:
         else:
             message = "Collecting %s"
             information = str(req.req or req)
+            # If we used req.req, inject requirement source if available (this
+            # would already be included if we used req directly)
+            if req.req and req.comes_from:
+                if isinstance(req.comes_from, str):
+                    comes_from: Optional[str] = req.comes_from
+                else:
+                    comes_from = req.comes_from.from_path()
+                if comes_from:
+                    information += f" (from {comes_from})"
 
         if (message, information) != self._previous_requirement_header:
             self._previous_requirement_header = (message, information)