Resolve the "improve for export *" TODO in SideEffectsFlagPlugin. A plain
`export *` into a side-effect-free module that is a pure single-star
passthrough (no own exports, exactly one `export *`) is equivalent to a
star into that module's source, so the passthrough can be skipped: move
the re-exported names' targets and repoint the star past the chain.

This lets namespace imports through such chains drop the intermediate
module (e.g. a barrel that only `export *`s another barrel). Named imports
were already resolved directly via per-name target moving; this extends
the same skipping to the star itself and to namespace consumers.

The detection short-circuits cheaply (leaf modules aren't passthroughs),
so the optimize-dependencies phase stays flat on star-heavy graphs.

When many modules import the same names from a shared barrel, the nested
import-specifier branch re-ran exportInfo.getTarget for each consumer even
though the side-effect-free resolution is idempotent within a pass. Cache
it per export info so each name resolves once instead of once per consumer.

Turns the getTarget portion from O(imports) into O(unique names): ~7-12%
off the plugin's own time on barrel-heavy graphs, neutral on graphs with
no repeated imports (the map lookup replaces an equivalent walk). The
cache is pass-scoped and released after optimizeDependencies.