[ty] add legacy namespace package support (#20897)
Detect legacy namespace packages and treat them like namespace packages
when looking them up as the *parent* of the module we're interested in.
In all other cases treat them like a regular package.
(This PR is coauthored by @MichaReiser in a shared coding session)
Fixes https://github.com/astral-sh/ty/issues/838
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Co-authored-by: Micha Reiser <micha@reiser.io>
