perf: look up the declaring module of imported constants without reference counting

Looking up only the module index of an imported constant, as `getModuleIdxFor?` does when the interpreter resolves the functions it calls, now returns a scalar from the lookup cache instead of the cached `Option (α × Nat)` result. Cache hits thus no longer increment and decrement the shared, multi-threaded result. `looksLikeCodegenName` also checks for a leading underscore before testing its prefixes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>