Make the validator compatible with re-included export-ignore directives

[raphaelstolt]

The validator should also be able to validate the following negated export-ignore directives.

# Default: nothing is included in the `composer archive` of the package.

* export-ignore

# Re-include only the files that actually belong to the distributed package below. 

# Top-level metadata.
composer.json    -export-ignore
LICENSE.md       -export-ignore

# Source code, packaged resources and CLI entry point.
/src             -export-ignore
/src/**          -export-ignore
/resources       -export-ignore
/resources/**    -export-ignore
/bin             -export-ignore
/bin/**          -export-ignore