Test cargo's exponential dependency blowup

[aleksator]

There is an issue with cargo not being able to perform dependency resolution in a reasonable time.
rust-lang/cargo#8539

Let's see if pubgrub can handle it by creating a test with a similar dependency structure.

[Eh2406]

This is a grate test case! It will need to wait for us to support most of Cargos features (multiple versions per crate, links attributes, and features) to be sure we are better.

[aleksator]

Indeed we should do that when making a case for inclusion in cargo.

I believe we can still test if we experience the blowup as is. Multiple versions per crate could be emulated by just renaming lib to lib_1 and lib_2 in the dependency tree, and the other 2 probably aren't central to the dependency resolution algorithm.