Add pip as a custom package via python -m ensurepip

pip is unusual among the custom packages because it ships inside CPython
itself, so it doesn't need a URL or archive. The handler bootstraps it
from the standard-library wheel via 'python3 -m ensurepip --upgrade' and
then self-upgrades to the latest version. Detection uses
'python3 -m pip --version' instead of a filesystem path.

formatted_packages.py

@@ -110,4 +110,5 @@ CUSTOM_PACKAGES: list[dict] = [
     },
     {"name": "nvm"},
     {"name": "pyenv"},
+    {"name": "pip"},
 ]