fix(rsc): fix plugin name in client-only error message

[hi-ogawa]

Description

Currently client-only error shows up as rsc:virtual-client-package:

[rsc:virtual-client-package] 'client-only' cannot be imported in server build (importer: '/home/hiroshi/code/others/vite-plugin-react/packages/plugin-rsc/examples/starter/src/root.tsx', environment: rsc)

We can hard-code server-only/client-only here to skip this step, so the error gets surfaced as:

[rsc:validate-imports] 'client-only' cannot be imported in server build (importer: '/home/hiroshi/code/others/vite-plugin-react/packages/plugin-rsc/examples/starter/src/root.tsx', environment: rsc)

This isn't fool proof though since other plugins might use this.resolve too. Eventually we should look into to surfacing errors later stage so we have a knowledge of module graph. Tracked in #863.