Auto merge of #3242 - jhbabon:cfgs-flags-per-package, r=alexcrichton

FIX: Call rustdoc test with the correct cfg flags of a package.

There was a situation in which if you you had a lib that depends on a package with features, whenever you ran the tests for the package the `rustdoc test` call was failing because it was called with the root `cfg` flags, not the package `cfg` flags.

This fix solves the issue by keeping track of the `cfg` flags per package, so the `rustdoc` command will be generated with the correct `cfg` flags.

Closes #3172