Support for pip install --target argument

[mpderbec]

We have some specific tooling that uses the -t flag in pip install to direct where we want pip to put the packages. (We aren't using virtual environments.)

Here's the help from pip for that argument:

-t, --target <dir>          Install packages into <dir>. By default this will not replace existing files/folders in <dir>. Use --upgrade to replace existing packages in <dir> with new versions.

Not sure how popular the argument is in pip land, but at least for us I've not found a way to soldier on without it.

[charliermarsh]

Do you mind chiming in here? #1517

Merging into that issue.