REPLExt: use Base.isaccessibledir rather than isdir in completions #4152

Job	Run time
docs	3m 44s
test (ubuntu-latest, x64, pkg.julialang.org, nightly)	17m 55s
test (ubuntu-latest, x86, pkg.julialang.org, nightly)	22m 43s
test (macOS-14, aarch64, pkg.julialang.org, nightly)	15m 26s
test (windows-latest, x64, pkg.julialang.org, nightly)	32m 26s
test (windows-latest, x86, pkg.julialang.org, nightly)	37m 34s
test (ubuntu-latest, x64, nightly)	20m 39s
	2h 30m 27s

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