fix(arborist): handle directories named yarn.lock - #9905

What / Why

A directory named yarn.lock currently makes Arborist's parallel lockfile read reject. The rejected Promise.all discards any package-lock contents read alongside it, and save() then fails with ERR_INVALID_ARG_TYPE because lockfile selection never completes.

Treat EISDIR as absence only for the optional yarn.lock input. All other yarn lock errors and every package-lock error retain the existing load-error handling.

The regression tests cover both an existing package lock, whose data must survive load and save, and a project without one, where save must create package-lock.json while leaving the yarn.lock directory intact.

References

Fixes #8782