Add a flag or command to build source packages

[jsoriano]

Originally proposed by @ruflin in elastic/package-spec#351 (comment)

Add a flag or subcommand to build source packages.

A source package would be a zip file that contains all the unmodified source files of a package, and would contain enough information to reproduce the build of the package.

It could be implemented as a flag for the elastic-package build command: elastic-package build --source. The generated source package could have an additional extension, something like .src.zip.

For packages in repositories with multiple packages, the source packages will also need to contain information that may not be in the directory of the package, as the license or the links tables.

We would also need to support signing these packages, in case we publish them in the registry.

[ruflin]

I filed a more specific issue for the sample data generation potentially using this feature here: #1640 It also proposes an alternative shortcut solution which doesn't mean we should stop any effort around source packages as I still see this as the final solution.