Filter resources for duplicate specs

As suggested by @ciaranmcnulty, this PR filters the array of Resources returned by the ResourceManager to remove Resources that are targeting the same spec. When a duplicate is found the decision of which one to keep is made based on the priority of the Resource's Locator.

The requirement for this has arisen from https://github.com/MageTest/MageSpec/issues/70, when we started to see that the PSR0Locator has started matching Zend namespaced specs that were previously only handled by the Magento specific Locators.
That said we have no real BC policy :-/