fs: defer path resolution in realpath functions #1876

Job	Run time
Build slim tarball	1m 29s
x86_64-darwin: with shared libraries	59m 13s
x86_64-linux: with shared libraries	29m 21s
aarch64-linux: with shared libraries	25m 42s
aarch64-darwin: with shared libraries	34m 38s
	2h 30m 23s

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