Parse 'Requires-Dist' markers

[zmanji]

Parse 'Requires-Dist' markers

[wickman]

After reading PEP345/426 more carefully, it probably makes sense for the PythonInterpreter ID_PY script to export all the defined environment markers for that interpreter:

• python_version: '{0.major}.{0.minor}'.format(sys.version_info)
• python_full_version: see definition below
• os_name: os.name
• sys_platform: sys.platform
• platform_release: platform.release()
• platform_version: platform.version()
• platform_machine: platform.machine()
• platform_python_implementation: platform.python_implementation()
• implementation_name: sys.implementation.name
• implementation_version: see definition below

def format_full_version(info):
    version = '{0.major}.{0.minor}.{0.micro}'.format(info)
    kind = info.releaselevel
    if kind != 'final':
        version += kind[0] + str(info.serial)
    return version

python_full_version = format_full_version(sys.version_info)
implementation_version = format_full_version(sys.implementation.version)

we can then just implement the PEP345/426 marker evaluator and have it run in the context of the PythonInterpreter.

the only challenge here is to build PEX files for other platforms we either need to get a handle to the remote interpreter or just fake it (e.g. if we see --platform=linux-x86_64 on the pex command line, we can't really actually support anything requiring platform_release/platform_version markers.)

[jsirois]

This state of affairs is now finally fully buttoned up with enhancements to PythonInterpreter: https://github.com/pantsbuild/pex/blob/3e71bc7896935a8177e9bbe9399769c3522528dc/pex/interpreter.py#L278-L286, AbbreviatedPlatform (--platform): https://github.com/pantsbuild/pex/blob/2b3bdee8cd99ffab0538a63fdd36de8de356f857/pex/targets.py#L214-L238 and the introduction of CompletePlatform (--complete-platform): https://github.com/pantsbuild/pex/blob/2b3bdee8cd99ffab0538a63fdd36de8de356f857/pex/targets.py#L246-L280

The abbreviated --platform case does not have enough information to form a full set of environment markers; so the surrounding code just hard fails iff solving of a resolve requires evaluating the missing env markers; otherwise, it proceeds correctly. This usually works since most env markers in the wild are python_version markers, but it fails when it should with the remedy of supplying a --complete-platform.