Fix PexInfo requirements using a non-deterministic data structure

[Eric-Arellano]

Problem

Even though PexInfo stores its requirements as an OrderedSet, when getting converted into a list in the dump() function, the requirements were losing their deterministic order for an unclear reason.

Specifically, the acceptance test diverged with PEX-INFO, with both files having different orders for their requirements.

# pex1
"requirements": ["six==1.12.0", "cryptography==2.6.1", "cffi!=1.11.3,>=1.8", "asn1crypto>=0.21.0", "pycparser"]

# pex2
["six==1.12.0", "cryptography==2.6.1", "asn1crypto>=0.21.0", "cffi!=1.11.3,>=1.8", "pycparser"]

Solution

Sort the requirements when being dumped to ensure we always have a consistent result.

We also make interpreter_constraints's output always be sorted, as it is internally stored as a set so by definition is not deterministic in ordering.

[Eric-Arellano]

The meaningful commit to review is 3834a42

I do not know why requirements is not deterministic when cast into a list, as it's an OrderedSet. I tried experimenting with this and could not figure it out. All I know is that this fixes the test and that the fix to interpreter_constraints does make logical sense.

[stuhood]

I'm fine with not understanding why order is being lost internally as long as the output ends up stable. Thanks!