The default library search for a package checks the lib/ and lib64/
directories for libraries before the root prefix, in order to save
time when searching for libraries provided by externals (which e.g.
may have '/usr/' as their root).
This moves that logic into the "find_libraries" utility method so
packages implementing their own custom library search logic can
benefit from it.
This also updates packages which appear to be replicating this logic
exactly, replacing it with a single call to "find_libraries".
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