Referencing resources outside directory containing the package document

[mattgarrish]

Probably low priority if we want to check this, but there is now authoring guidance not to reference resources outside the directory containing the package document because not all reading systems allow this (i.e., the resources won't be used/rendered).

Would only be an informative message if it is checked.

See w3c/epub-specs#1687

[rdeltour]

Moving this one out of v5.0.0, this is low priority.