Fix pex3 lock sync for split universal locks.

[jsirois]

Fixes #3194

[jsirois]

N.B.: Before the fix, this failed with:

ERROR: Given the lock requirements:
cowsay<6; python_version < "3"
cowsay==6; python_version >= "3"

The following lock update constraints could not all be satisfied:
cowsay==6

Encountered 1 error updating /tmp/pytest-of-jsirois/test_sync_split_universal_lock0/lock.json:
1.) cp314-cp314-manylinux_2_43_x86_64: pid 160054 -> /tmp/pytest-of-jsirois/test_sync_split_universal_lock0/pex-root/venvs/3/8bd2f07c0ed6715bf11b8f6442a9a826ea7fcfd6/a96ab6db284dbfcf987e72d97602026d5f42ecb0/bin/python /tmp/pytest-of-jsirois/test_sync_split_universal_lock0/pex-root/venvs/3/8bd2f07c0ed6715bf11b8f6442a9a826ea7fcfd6/a96ab6db284dbfcf987e72d97602026d5f42ecb0/pex --disable-pip-version-check --exists-action a --no-input --no-python-version-warning --log /tmp/pex-pip-log.sb_rinnr/pip.log -q --cache-dir /tmp/pytest-of-jsirois/test_sync_split_universal_lock0/pex-root/pip/3/24.1/pip_cache install --no-clean --dry-run --ignore-installed --report /tmp/pytest-of-jsirois/test_sync_split_universal_lock0/pex-root/downloads/1/.tmp/resolver_report.rn6al2l6/universal-linux-and-mac-and-windows-84eb388031e26712b5bd51cbb3789f9c24c9a5d1/pip-report.json --constraint /tmp/lock_update.vcgmb9ae.constraints.txt cowsay<6; python_version < "3" cowsay==6; python_version >= "3" --retries 5 --timeout 15 exited with 1 and STDERR:
pip: ERROR: Cannot install cowsay<6 because these package versions have conflicting dependencies.
pip: ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/topics/dependency-resolution/#dealing-with-dependency-conflicts
pip:  
pip:  The conflict is caused by:
pip:      The user requested cowsay<6
pip:      The user requested (constraint) cowsay==6
pip:  
pip:  To fix this you could try to:
pip:  1. loosen the range of package versions you've specified
pip:  2. remove package versions to allow pip to attempt to solve the dependency conflict

This is much as the OP in #3194. That OP case succeeds with the following post-fix:

:; python -mpex.cli lock sync \
  "torch>=2.9.1,<3.0.0; sys_platform != 'darwin'" \
  "torch>=2.9.1,<3.0.0; sys_platform == 'darwin'" \
  --index "https://pypi.org/simple/" \
  --index "pytorch=https://download.pytorch.org/whl/cpu" \
  --source "pytorch=torch" \
  --indent 2 \
  --lock default.lock \
  --style universal \
  --interpreter-constraint "==3.12.*"
No updates for lock generated by universal (platform_system == "Linux" or platform_system == "Windows") and python_full_version == "3.12.*".
No updates for lock generated by universal platform_system == "Darwin" and python_full_version == "3.12.*".