[Distutils] deprecating pip install --target

[Robert Collins]

This is fairly broken - it doesn't handle platlib vs purelib (see pip
PR 3450), doesn't handle data files, or any other layout. Donald says
pip uses it to maintain the _vendor subtrees only, which doesn't seem
like a particularly strong use case.

Certainly the help description for it is misleading - since what we're
actually doing is copying only a subset of what the package installed
- so at a minimum we need to be much clearer about what it does.

But, I think it would be better to deprecate it and remove it... so
I'm pinging here to see if anyone can explain a sensible use case for
it in the context of pip :)

-Rob



-- 
Robert Collins <rbtcollins at hpe.com>
Distinguished Technologist
HP Converged Cloud