WIP Feature/pip wheels windows

[mottosso]

Based on https://github.com/lambdaclan/rez/commits/feature/pip-install-modern.

Specific to Windows, but also affects Linux, see below.

Problem

Some packages - like lxml, numpy and PySide - have wheels that avoid a need to compile, solving primarily for Windows users - whom may not have the necessary compilers installed for building those.

The cost however is that this approach couldn't separate the source, header and lib files, like a vanilla rez-pip would.

Solution

$ rez pip --install lxml --use-wheel

Add rez pip --use-wheel to recommend pip to use wheels when possible, and fall back to building otherwise (default pip behavior). Backwards compatibility is maintained by having the argument be optional.

Test

Confirmed to work with these, both wheel and non-wheels alike.

$ rez pip --install --release --use-wheel ^
    Cython ^
    Jinja2 ^
    MarkupSafe ^
    Pillow ^
    PySide ^
    Qt.py ^
    blockdiag ^
    certifi ^
    excel ^
    funcparserlib ^
    lockfile ^
    lxml ^
    ordereddict ^
    pyblish-base ^
    pyblish-lite ^
    pyblish-maya ^
    pythonnet ^
    pywin32 ^
    setuptools ^
    urllib3 ^
    webcolors ^
    xlrd ^
    six

Moving forward

--use-wheel doesn't actually enforce the use of wheel; it merely calls pip install without additional arguments, which just so happens to default to using wheels. The question is whether --use-wheel should force a wheel, and fail if none is found. Or if --use-wheel is the best name for this argument.

I also removed the ability to pick a pip version; not because it isn't important, but because it simplified the implementation and I honestly couldn't see a need to separate between both Python and Pip versions; whichever pip is available as a package during rez pip would get picked up by this approach - which in a nutshell is python -m pip. I'm probably missing something though, so something to look out for.

[mottosso]

Superseded by #614