[ty] Limit wildcard exports using literal `__all__` (#28972)
## Summary
When a module imports a large namespace and then defines a small
`__all__`, we currently include every imported name when another module
uses a wildcard import. Packages with many such modules can create
hundreds of thousands of unnecessary definitions and reachability
constraints.
This PR instead recognizes an unchanged literal `__all__` before
semantic indexing and limits the possible exports to its names. Other
uses of `__all__`, including mutations and later wildcard imports, fall
back to the existing analysis. So this fixes narrow (but non-contrived)
cases.
For the `pulumi-cloudflare` reproduction in
[astral-sh/ty#4606](https://github.com/astral-sh/ty/issues/4606), peak
memory goes from 5,649-6,344 MiB to 453-459 MiB.
This also changes some diagnostics by avoiding cycles during import
resolution. Previously, determining the contents of `__all__` could
require type inference that depended on imports we were still resolving,
leaving some types as `Divergent`. Reading the literal directly breaks
those cycles, so we resolve more types successfully. This removes
spurious warnings and enables checks that were previously skipped
because the relevant types were unresolved.
