pex fails to resolve wheels when a sdist is present in the cache

[sitaktif]

When the source dist for a package is present in the cache, pex will not try to resolve the wheel for that package. Removing the source dist from the cache will allow pex to work as expected.

# Clear the cache
rm ~/.pex/build/*

# Download the source dist and put it into the cache
curl -Lo ~/.pex/build/cryptography-2.3.1.tar.gz https://pypi.org/packages/22/21/233e38f74188db94e8451ef6385754a98f3cad9b59bedf3a8e8b14988be4/cryptography-2.3.1.tar.gz#sha256=8d10113ca826a4c29d5b85b2c4e045ffa8bad74fb525ee0eceb1d38d4c70dfd6

# Try to create the pex (optionally provide a platform that your local machine cannot build for)
pex cryptography==2.3.1 -vvv --platform linux_x86_64-cp-27-cp27mu -o foo.pex

# Notice that it tries to build the source dist into a pex (and if you are targeting a different platform,  fails with):
# pex.resolver.Untranslateable: Package
# SourcePackage('file:///Users/rchossart/.pex/build/cryptography-2.3.1.tar.gz')
# is not translateable by ChainedTranslator(WheelTranslator, EggTranslator, SourceTranslator)

# Remove the source dist from the cache
rm ~/.pex/build/cryptography-2.3.1.tar.gz

# Try to create the pex again
pex cryptography -vvv --platform linux_x86_64-cp-27-cp27mu -o foo.pex

# Notice that it succeeds
ls foo.pex
# --> foo.pex

Expected behaviour:

I would expect pex to first look for an existing wheel in pypi before trying to create one from the local source distribution.