Consecutive expand-file-name and recursive apply's can be expensive, so
the function has been simplified to rely more on file-name-concat. This
does change one trait about it, however: absolute paths in SEGMENTS no
long reroot the whole path, and are concatenated as ordinary file
segments.
The performance benefit is more pronounced on Emacs 28+, and will be
even more so when Doom later starts byte-compiling its libraries.
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