Lack of complete manylinux1 wheel support in pex forces pants workarounds for plugins

[jsirois]

Pex doesn't yet fully support manylinux1 wheels; yet, pants is forced to release one to satisfy pypi hosting constraints. When the pants plugin resolver runs, it uses pex resolution machinery and chokes trying to resolve pantsbuild.pants as a transitive dependency of the plugin. This is currently worked around by deploying a pantsbuild.pants sdist to pypi along-side the platform-specific wheels. The sdist is bogus since it contains a native_engine.so for only one platform and the workaround is only effective for pip installs and then only because the pip installed pantsbuild.pants wheel is earlier in the PYTHONPATH than the pantsbuild.pants sdist resolved as plugins' transitive dependency.

Remove deployment of sdists alongside wheels from build-support/bin/release.sh when we upgrade to pex with full manylinux1 support.

See:

• Support manylinux1 wheels pex-tool/pex#281
• Add manylinux wheel support and fix a few bugs along the way pex-tool/pex#316

[jsirois]

An important note - I setup a travis-ci job for the https://github.com/pantsbuild/setup repo to verify the workaround works under pip for both OSX and linux: https://travis-ci.org/pantsbuild/setup/builds/284937425

[kwlzn]

I have a forked-and-extended variant of pex#316 95% of the way here: pex-tool/pex#399

if this is a big enough problem, I can probably justify spending some time to get that out with the team.

[kwlzn]

pex provides full manylinux support as of 1.4.0, so this should now be unblocked.

[jsirois]

A late note that these workarounds led a user to reasonably set up their own repo only containing sdists and then attempt to run pants (via pantsbuild/setup script). This failed as described in #5882 and fixing this issue will remove the temptation to try to build pants from a bogus pantsbuild.pants sdist since we'll no longer publish one of those.