fs: fix errorOnExist behavior for directory copy in fs.cp #2378

Job	Run time
Build slim tarball	1m 33s
aarch64-linux: with shared libraries	21m 57s
x86_64-linux: with shared libraries	26m 37s
aarch64-darwin: with shared libraries	23m 31s
x86_64-darwin: with shared libraries	34m 38s
	1h 48m 16s

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