bug#22013: python2 versions of packages failing as inputs

[Efraim Flashner]

Guix build python2-cryptography builds fine, guix build python2-oauthlib
rebuilds python2-cryptography, and fails to build. Python2-cryptography has
an additional input of python2-ipaddress which doesn't get carried along when
python2-cryptography is an input and not the final build.

My guess is that package-with-python2 needs some tweaking. It looks like
python-oauthlib uses python-cryptography, python2-oauthlib uses
(package-with-python2 python-cryptography) instead of python2-cryptography.

I've enclosed `guix graph python2-cryptography python2-oauthlib` as a
visualization of the bug.

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