pip refactoring will break pipdeptree

@dstufft has decided to move around the imports in pypa/pip#4700, which will necessitate adjustments to the imports in pip 10.0.

[naiquevin]

Yeah, pipdeptree is pretty much a hack because it relies on pip as a library and uses it's internal api. I knew we'd run into this situation some day! But if it's just code being moved around, I think it should be a fairly straightforward change to handle imports for different versions of pip using a simple try..catch block.

Thanks for informing 👍 I'll keep track of that PR in the pip repo.

[dstufft]

Yea, basically no code changes, just shuffling everything into pip._internal to try and make it more obvious that the APIs are internal, to hopefully prevent some people from accidentally using them unawares of that fact.