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

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 rustdoc
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.