#54739-related fixes for loading stdlibs (#54891)
This fixes a couple unconventional issues people encountered and were
able to report as bugs against #54739
Note that due to several bugs in REPLExt itself
(https://github.com/JuliaLang/julia/issues/54889,
https://github.com/JuliaLang/julia/issues/54888), loading the extension
may still crash julia in some circumstances, but that is now a Pkg bug,
and no longer the fault of the loading code.
