--prefix doesn't work for OSX Framework Python

If a user provides --prefix to setuptools, setuptools adds a site directory for ${prefix}/lib/python{version}/site-packages on OSX -

https://github.com/pypa/setuptools/blob/6547430bf289c7285498303ea77eef56702c3a95/setuptools/command/easy_install.py#L1360

Problem is, for OSX Framework Python this isn't true. The site-packages wind up being at ${prefix}/lib/python/site-packages -https://github.com/python/cpython/blob/b505c47044fb5b8f03b9708ee1f210a4c935baf5/Lib/site.py#L267 (which then calls into https://github.com/python/cpython/blob/a6431f2c8cf4783c2fd522b2f6ee04c3c204237f/Lib/sysconfig.py#L75)

This is causing setuptools to fail when installing packages into a prefixed directory. For example (on OSX 10.11.15, python 2.7 installed via brew)

export PYTHONUSERBASE=$(mktemp -d)
export PYTHONPATH="$(python -m site --user-site)/"
wget https://pypi.python.org/packages/1b/4f/e52b81c47396668deb139b628f4ebb499b3cd39fc05382851fa985d0b642/setuptools-24.3.0.tar.gz#md5=55d77ca2b1f783a71e330b0878da29ec
tar xzf setuptools-24.3.0.tar.gz
(cd setuptools* && python setup.py --quiet install --prefix=$PYTHONUSERBASE)
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.
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error: bad install directory or PYTHONPATH

You are attempting to install a package to a directory that is not
on PYTHONPATH and which Python does not read ".pth" files from.  The
installation directory you specified (via --install-dir, --prefix, or
the distutils default setting) was:
.
.
.

Start in setuptools/command/easy_install.py at the linked prefix site-directory logic, then compare it with the linked CPython Lib/site.py and Lib/sysconfig.py behavior for macOS Framework Python. Reproduce the failure with the provided PYTHONUSERBASE, PYTHONPATH, and setup.py install command; done means installation into a prefixed directory no longer reports a bad install directory or PYTHONPATH.