Attempt "cross-builds" of sdists for foreign platforms.

[jsirois]

The "cross-building" is in scare-quotes because this is not actually
cross-building, it is just attempting a build of the sdist, and, if
successful, seeing if the resulting wheel matches the foreign platform.
This enables sdist-only projects to be used in foreign platform Pex
operations when it turns out the sdist is multi-platform.

Closes #2073

[jsirois]

This test changed substantially since ansicolors is now an example of a distribution which is "cross-buildable" for foreign platforms, but it did so to reflect the actual original issue: #1355

That issue is now verified directly and exactly.

[jsirois]

This is a bit of a graduation for Pex. As of this change, which many prior changes over the last 2-3 years have laid the groundwork for, I see no more buggy-by-design behaviors left over from the Twitter years that can be fixed without breaking people. Time permitting, Pex 3.0.0 is ready to be prepared for (writing a real doc site) and cut.

[jsirois]

I'm on Linux.

Old:

$ python -mpex.cli lock create --style universal --platform macosx-10.13-x86_64-cp-36-m psutil==5.7.0 -o psutil.lock.json
Failed to resolve compatible artifacts from lock for 1 target:
1. cp36-cp36m-macosx_10_13_x86_64:
    Failed to resolve all requirements for abbreviated platform cp36-cp36m-macosx_10_13_x86_64:

Configured with:
    build: False
    use_wheel: True

Dependency on psutil not satisfied, 1 incompatible candidate found:
1.) psutil 5.7 (via: psutil==5.7.0) does not have any compatible artifacts:
    https://files.pythonhosted.org/packages/0b/6b/f613593812c5f379c6d609bf5eca36a409812f508e13c704acd25712a73e/psutil-5.7.0-cp27-none-win32.whl
    https://files.pythonhosted.org/packages/4f/3c/205850b172a14a8b9fdc9b1e84a2c055d6b9aea226431da7685bea644f04/psutil-5.7.0-cp36-cp36m-win_amd64.whl
    https://files.pythonhosted.org/packages/54/25/7825fefd62635f7ca556c8e0d44369ce4674aa2ca0eca50b8ae4ff49954b/psutil-5.7.0-cp37-cp37m-win32.whl
    https://files.pythonhosted.org/packages/63/d5/f34a9433a0299d944605fb5a970306a89e076f5412164179dc59ebf70fa9/psutil-5.7.0-cp38-cp38-win32.whl
    https://files.pythonhosted.org/packages/65/c2/0aeb9f0cc7e4be2807aa052b3fd017e59439ed6d830b461f8ecb35b2f367/psutil-5.7.0-cp35-cp35m-win_amd64.whl
    https://files.pythonhosted.org/packages/74/e6/4a0ef10b1a4ca43954cd8fd9eac02cc8606f9d2a5a66859a283f5f95452b/psutil-5.7.0-cp35-cp35m-win32.whl
    https://files.pythonhosted.org/packages/79/b1/377fa0f28630d855cb6b5bfb2ee4c1bf0df3bc2603c691ceefce59a95181/psutil-5.7.0-cp27-none-win_amd64.whl
    https://files.pythonhosted.org/packages/86/f7/385040b90dd190edc28908c4a26af99b00ae37564ee5f5c4526dc1d80c27/psutil-5.7.0-cp37-cp37m-win_amd64.whl
    https://files.pythonhosted.org/packages/86/fe/9f1d1f8c1c8138d42fc0e7c06ca5004e01f38e86e61342374d8e0fa919e4/psutil-5.7.0-cp38-cp38-win_amd64.whl
    https://files.pythonhosted.org/packages/c4/b8/3512f0e93e0db23a71d82485ba256071ebef99b227351f0f5540f744af41/psutil-5.7.0.tar.gz
    https://files.pythonhosted.org/packages/c9/37/b94930ae428b2d67d505aecc5ba84c53a0b75479a8a87cd35cc9a2c6eb7e/psutil-5.7.0-cp36-cp36m-win32.whl

New:

$ python -mpex.cli lock create --style universal --platform macosx-10.13-x86_64-cp-36-m psutil==5.7.0 -o psutil.lock.json
No pre-built wheel was available for psutil 5.7.0.
Successfully built the wheel psutil-5.7.0-cp310-cp310-linux_x86_64.whl from the sdist psutil-5.7.0.tar.gz but it is not compatible with the requested foreign target abbreviated platform cp36-cp36m-macosx_10_13_x86_64.
You'll need to build a wheel from psutil-5.7.0.tar.gz on the foreign target platform and make it available to Pex via a `--find-links` repo or a custom `--index`.

[benjyw]

Great! A couple of minor nits that are not worth fixing and triggering another CI run, but maybe worth tacking on to a future unrelated change.

[jsirois]

Hrm, it looks like these 2 Python 3.5 failures are not bitrot. Main is definitely green for them. I'll circle back in the am and figure these two out before proceeding.

[jsirois]

Ok, yeah - this caught a bug. When I eliminated iter_platform_args(), that killed sending Pip the --platform ... --implementation ... --python-version ... --abi ... quad. The foreign_platform patches cover --platform ... --implementation ... --abi ..., aka: wheel tag matching, but they do not cover --python-version ..., aka Requires-Python matching. Pip requires you send the quad as a whole; so the --python-version ... / Requires-Python matching must be patched too. That patching is already implemented in the locker patches; so I'll need to share that code here for foreign platform resolves.

[jsirois]

Ok, I've reverted the test skips and updated the PR to include patching Requires-Python for foreign platform resolves. The PR update is large, but its mainly shuffling. The code to patch Requires-Python already existed to support universal lock resolves and so 95% of the delta here is extracting that bit of patching and re-arranging for re-use in the foreign platform patches.