* drop duplicate mkdir for .heroku

* simplify linking and moving of python and source

* steps/python needs its own symlink setup

* fix symlink destinations for vendor and venv

* always symlink python

This would REQUIRE that the latest versions are placed into:
https://github.com/heroku/heroku-buildpack-python/tree/master/vendor

https://pypi.python.org/pypi/pip
https://pypi.python.org/pypi/setuptools

This reverts commit 75870dd4, reversing
changes made to 0cf2e284.

Preserve LIBRARY_PATH and LD_LIBRARY_PATH.
Related heroku/heroku-buildpack-python#287

Update setuptools from 23.x.x to 25.x.x.

* $ curl
https://pypi.python.org/packages/e7/a8/7556133689add8d1a54c0b14aeff0acb0
3c64707ce100ecd53934da1aa13/pip-8.1.2.tar.gz -o pip-8.1.2.tar.gz
* $ curl
https://pypi.python.org/packages/9f/7c/0a33c528164f1b7ff8cf0684cf88c2e73
3c8ae0119ceca4a3955c7fc059d/setuptools-23.1.0.tar.gz -o
setuptools-23.1.0.tar.gz
* $ ls
* $ rm pip-8.1.1.tar.gz
* $ rm setuptools-20.4.tar.gz

`PKG_CONFIG_PATH` variable.

https://pythonhosted.org/setuptools/history.html
https://bitbucket.org/pypa/setuptools/branches/compare/20.2.2%0D19.6

The release on PyPI isn't signed, though the MD5 of this archive matches
that on the PyPI package page:

$ md5sum setuptools-20.2.2.tar.gz
bf37191cb4c1472fb61e6f933d2006b1 *setuptools-20.2.2.tar.gz

https://pip.pypa.io/en/stable/news/
https://github.com/pypa/pip/compare/8.0.2...8.1.0

Archive verified via:
https://pypi.python.org/packages/source/p/pip/pip-8.1.0.tar.gz.asc

And Donald's key from:
https://pypi.python.org/security

$ gpg --verify pip-8.1.0.tar.gz.asc pip-8.1.0.tar.gz
gpg: Signature made Sat, Mar  5, 2016  4:57:30 PM GMT using RSA key ID 3372DCFA
gpg: Good signature from "Donald Stufft (dstufft) <donald@stufft.io>"
gpg:                 aka "Donald Stufft (dstufft) <donald@python.org>"
...
Primary key fingerprint: 7C6B 7C5D 5E2B 6356 A926  F04F 6E3C BCE9 3372 DCFA