change default LOAD_PATH and DEPOT_PATH (#26804, fix #25709)
also:
- rename "site" directory to "stdlib" since that's what it is now
- use JULIA_LOAD_PATH as-is instead unconditionally appending the
system package and stdlib directories to it
- change default DEPOT_PATH to include system paths: one for
arch-specific and one for arch-independent packages
- delete comment about bundled code going in a versioned directory
as it no longer applies since installed packages can and should
be shared across different Julia versions
- update Pkg3 and tests to work correctly when the stdlib directory
isn't always included in LOAD_PATH
