Auto merge of #1828 - fhahn:multiple-package-parameters, r=alexcrichton

Auto merge of #1828 - fhahn:multiple-package-parameters, r=alexcrichton

This PR for #1528 is still pretty rough (and so far only added multiple package support for tests), but I wanted to make sure the overall approach is fine.

I have made some progress and cargo is now able to compile and execute tests for multiple packages.

In order to execute the doc tests for multiple packages as well, I added `to_doc_test` to `Compilation` (used [here](https://github.com/fhahn/cargo/commit/8b8df3e8d5abfa638852f55730d2f3cb4eec66bc#diff-417e085367d0ce027505dfaa26a089d3R33)). Previously it only executed the doc tests for `Compilation.package`. Another option would be to change `Compilation.package` to be a `Vec<Package>`, because in order to execute the tests of multiple packages, multiple packages are compiled as "top level package". But this would require a changes at a couple of other places.