Add Cliver::detect

[yaauie]

WIP notes

Sometimes just making sure that the first executable on your path is the right version isn't enough; we should be able to look for an executable that satisfies the requirements, not just make sure that the first one on the path is ok. A perfect example of this is python, with the 2/3 split, where many users must have both installed.

The interface should be pretty similar to the existing Cliver::assert, but should have an explicit return value of the full path to the executable. If no appropriate executable can be detected, this will raise like Cliver::assert does, except that the message needs to have information about many found-and-not-good-enough binaries.

executable_path = Cliver::detect(executable, *requirements, options = {})