Invalid vendir config is accepted

[Zebradil]

See #274 (comment)

It is possible to supply myks with an invalid vendir config which will be accepted and processed with incorrect result.

Vendir doesn't allow managing overlapping paths in different directories (as per vendir spec). But because myks doesn't run vendir on the original config but iterates over directories in it, this misconfiguration is not caught by vendir's validation. This should be corrected.