fix: parallel file validation for a datapackage

[pierrecamilleri]

• fixes Adding --parallel CLI flag breaks validation #1644

I think I have identified what breaks the parallel validation of a datapackage : in validator.py, Resource is imported as a type, but then used as a class to retrieve a class method. (Resource.from_descriptor)

Importing Resource leads to a circular import. A small workaround is to import Resource locally, but this is not very satisfactory.

Todo : 

• Test that it works correctly
• Check why the tests did not intercept this error
• See if I can find a more elegant way to deal with the circular import. I actually don't see the benefit of having this validator.py class instead of dispatching the methods to Resource and DataPackage directly, but I may have missed something.