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.
It's a BC break to modify this.

You could introduce a PrioritisedResourceInterface and do a type check inside the Manager, treating Resources that don't implement the interface as priority 0