fs: fix errorOnExist behavior for directory copy in fs.cp #1418

Job	Run time
Build slim tarball	1m 36s
x86_64-darwin: with shared libraries	1h 1m 17s
aarch64-darwin: with shared libraries	28m 46s
aarch64-linux: with shared libraries	26m 26s
x86_64-linux: with shared libraries	29m 34s
	2h 27m 39s

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