Teach find_(library|file|path) to get prefixes from PATH (#15370)
The find_package command already knows how to compute installation prefixes from PATH. Use the same approach to establish prefixes for find_library, find_file, and find_path to use to look in directories like "<prefix>/lib[/<arch>]" and "<prefix>/include" for libraries and headers. This will reduce the amount of configuration end users need to do to establish a work environment rooted under a specific prefix.
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