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)