Skip sort, native extension probe, and per-symlink realpath
when called with --bare --skip-aliases (the rehash case).
Use readlink to distinguish internal aliases (relative target)
from external installs (absolute target).
In MacOS, speeds up rehash by 5.2 ms (7%) in empty environment,
55.9 ms (33%) in a regular environment (3 ver, 12 venvs, 320 bins),
93.7 ms (15%) in a large environment (6 ver, 60 venvs, 1466 bins)
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