Include source code in the package tarballs for GPL packages

The GPL and other similar licenses require the source code to be distributed with the binary or equivalently alongside the binary if the binary is distributed. We distribute binaries of GPL libraries via anaconda.org, but the source code associated with that binary is not available in the tarball or alongside the tarball.

Some references:

- http://www.gnu.org/licenses/gpl-faq.en.html#GPLRequireSourcePostedPublic
- 4.1.1 Option (a): Source Alongside Binary https://www.softwarefreedom.org/resources/2008/compliance-guide.html
- https://opensource.stackexchange.com/questions/5106/can-i-distribute-unmodified-gplv2-binaries-without-the-source-code

A solution would be to include the source tarball downloaded in the build process in the resulting conda package tarball. This could be be done for all packages regardless of the license so you don't have to determine which licenses require this.

Alternatively, anaconda.org should also have a source tarball along side each unique release, similar to the source channels available with Ubuntu.