Ensure only one wheel for each vendored package in `verify_ensurepip_wheels.py`

[ambv]

We've had two cases of old wheels not being removed while a new one is added. This lead to CI failures on the affected branches, which required a fix afterwards like gh-108998.

Let's fix it by making the tool complain if there's more than one wheel present.

Reported by @edmorley.

Linked PRs

• gh-109002: Ensure only one wheel for each vendored package #109003
• [3.12] gh-109002: Ensure only one wheel for each vendored package (GH-109003) #109005
• [3.11] gh-109002: Ensure only one wheel for each vendored package (GH-109003) #109006
• [3.10] gh-109002: Ensure only one wheel for each vendored package (GH-109003) #109007
• [3.9] gh-109002: Ensure only one wheel for each vendored package (GH-109003) #109008
• [3.8] gh-109002: Ensure only one wheel for each vendored package (GH-109003) #109009