Checker: error when named import does not exist in module

[milkyskies]

When importing a named export that doesn't exist in a module (e.g. import { Markdown } from "react-markdown" when the package only has a default export), the compiler should error instead of silently generating broken code.

Currently requires tsgo to resolve npm package exports. Without tsgo, all npm imports are unresolved and the checker can't verify they exist.

Blocked by tsgo integration for npm packages.