allow finding stdlibs that are normal packages in the manifest but are now stdlibs (#60061)
This helps handling manifest from earlier julia versions. This is kind
of the `locate` version of the `identify` fallback in
https://github.com/JuliaLang/julia/pull/56148.
Need to write a test when this happens (and verify that this fix works).
Noted in a slack #gripes comment.
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Co-authored-by: KristofferC <kristoffer.carlsson@juliacomputing.com>
Co-authored-by: Max Horn <max@quendi.de>