Fixed performance bug in UIOP:DIRECTORY-FILES

[Faré]

I bumped ASDF to 3.1.7.1 after I fixed a performance bug in
directory-files that made it quadratic instead of linear, due to some
remove-duplicates used for logical-pathnames.
The fix was to only use this quadratic behavior for LPNs. If you use
LPNs, you deserve to lose on large directories. LPNs are for legacy
Lisp code with relatively few files, only.

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