Fix confusion of responsibilities between Internal\Path\Value\Path and Internal\Helper\PathHelper

[TravisCarden]

Internal\Path\Value\Path and Internal\Helper\PathHelper contain duplicative and inconsistent logic, suggesting a violation of the Single Responsibility Principle (SRP). This is apparent from the fact that each has an ::isAbsolute() method, but the one on Path hands protocols, whereas the one on PathHelper does not. All protocol-handling should be moved to Path. In fact, most path computation should be centralized in PathHelper, and Path should be trimmed to focus on the responsibilities of a true value object.