Issues with global imports before executing a doctest

[Erotemic]

In instances where the first line of a doctest is a REQUIRES statement like a module

            >>> # xdoctest: +REQUIRES(module:kwarray)

It should check if that requirement is satisfied before doing the global import. Furthermore, it should determine if any blocks of code are satisfied before doing a global import.

This is causing an issue in netharn.examples.object_detection, where there is a global kwimage import, but we are testing in minimal dependecy mode. All of the tests are marked as requiring the kwimage module. So it would be best of none of these tests were executed.

Additionally, there should be a way to globally disable a module from being included in xdoctest.

Additionally, when the global import fails, this breaks the entire harness run, not just the single doctest. That should be fixed such that only that doctest fails.