`FullPaths` will panic if the file doesn't exist

[Tatskaari]

FullPaths will panic if the file doesn't exist. Rewriting the BuildInput implementation to return errors would probably be better, but I felt it would be overkill for now. This function should also work predictably well as there should only be one path returned, which the recover will catch if not resolved.

Originally posted by @tiagovtristao in https://github.com/thought-machine/please/pull/2130#discussion_r781328394

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