rez-pip-integration

Converting pip packages into rez packages — rez-pip usage, install vs release vs custom prefix, choosing and pinning the python version, how rez-pip picks which pip to run, the python-MAJOR.MINOR dependency it writes, extra pip arguments, the pip_install_package API and InstallMode, and the failures that account for most broken conversions. Use when the user wants a PyPI package available as a rez package, or asks why a converted package will not resolve. Covers Rez 3.4.0.