Ignores dependencies only present in "_test" packages

[mstine]

I'm using Ginkgo, but this would be true for regular tests as well I reckon.

When running godep save, the resulting json does not include any dependencies that are only present in "_test" packages.

e.g. my code is in package foo, tests are in foo_test.

If foo_test imports github.com/onsi/ginkgo, but no non-"_test" package imports it, then ginkgo will be excluded from Godeps.

I've worked around this by moving everything out of "_test" packages for now, but that's not an ideal solution.

Thanks!