Switch to a dedicated tool for Pipenv’s own vendoring process

[uranusjr]

pip recently moved their vendoring tasks to a separate project (pypa/pip#7485). We might want to reuse that since Pipenv’s vendoring tasks are largely modelled after pip.

cc @pradyunsg @techalchemy

[pradyunsg]

Yeah, I'll file a PR for this soon-ish.

First though, I need to improve the messaging based on the latest comment by @dstufft to discourage use by folks who don't definitely need it (basically everyone except pip and similar tools), since we don't want to make life harder for redistributors who don't like vendored packages.

[techalchemy]

Since I’m on both sides of this issue now as a maintainer and a redistributor I’m in favor. Just wrapping up vendored updates hopefully so we can cut a release. I believe I can start handing out access as well. Then I am pretty fine with any changes.