Add flit-core as wheel to pip bundle

[ivanovac]

Add flit-core as wheel to pip bundle (required by pip 26.x build backend)

Problem

pip 26.x switched its build backend from setuptools to flit-core, requiring flit-core>=3.11,<4 as a build dependency. When the Python buildpack installs pip from the bundle tarball using:

pip install --no-index --find-links=/tmp/pip

pip tries to install its build dependencies from /tmp/pip, but the bundle only contained pip's own source tarball — not flit-core. This caused staging to fail with:

ERROR: Could not find a version that satisfies the requirement flit-core<4,>=3.11 (from versions: none)
ERROR: No matching distribution found for flit-core<4,>=3.11

Fix

Add flit-core>=3.11 to ExtraDeps in PipRecipe. ExtraDeps are downloaded without --no-binary :all:, so flit-core is fetched as a pre-built wheel (flit_core-x.x.x-py3-none-any.whl). This breaks the circular dependency (flit-core source requires flit-core to build) and makes it available in /tmp/pip for pip to use as its build backend.

Testing

Rebuild the pip 26.x bundle using binary-builder and verify staging a Python app with PIP_VERSION=latest succeeds on cflinuxfs4/cflinuxfs5.

Fixes: cloudfoundry/python-buildpack#1200