Report a diagnostic for a URL specifier in `@value ... from`

Summary

css-modules-kit reports no diagnostic for @value ... from with a URL specifier, although bundlers cannot import tokens from a URL. composes ... from with a URL specifier is reported, but the two should be handled consistently.

Details

Current diagnostics of the checker:

Behavior of css-loader 7.1.5 with webpack 5.111.0 (default settings, without experiments.buildHttp):

Both failures have the same message:

Module build failed (from ./node_modules/css-loader/dist/cjs.js):
Error: Can't resolve './https://example.com/a.module.css' in '<dir>'

css-loader converts both @value ... from and composes ... from into ICSS :import("<URL>") and resolves them in the same code path. That path has no check for URLs, so the URL is passed to the resolver like a path of a local file. Only @import skips URLs explicitly.

packages/core/src/checker.test.ts already has a test for this case with // TODO: Report diagnostics.

Expected behavior

• @value ... from '<URL>' (named token importer) is reported.
• composes ... from '<URL>' (external token reference) is reported in the same way.
• @import '<URL>' (all token importer) stays unreported.

Open question: which message to report.

• Reuse Cannot import module '<URL>'. It matches css-loader, where the failure is a plain resolution error.
• Add a message that tells the reason, such as Cannot import tokens from the URL '<URL>'., and use it for both constructs.

🤖 Generated with Claude Code

Start with the URL-specifier test and its TODO in packages/core/src/checker.test.ts, then trace the checker path that handles @value and composes imports. Decide on the diagnostic wording for both constructs while leaving URL @import unreported. Done means the named test covers the expected diagnostics and the relevant checker tests pass.