[Change Proposal] Add different validation modes for source and built packages

[jsoriano]

Now we are using the very same spec to validate source and built packages, but the content can be actually different. For example dev files or external fields can be found in source packages but are not expected to be on built packages. Adding two validation modes can allow us to be more strict with the content and format of built packages. It can also help on testing package build implementations like the one in elastic-package.

We would need to:

• Expose in the validator package an interface to select the validation mode.
• Define somehow in the spec the contents for each mode.

[jsoriano]

This can be also relevant for link files, introduced in #888.

In this case link files should be allowed in the source, but not in built packages.