[core] Improve DX of importing Base UI components

[michaldudak]

Changed how components are exported. Previously we had two index files: index.ts containing unprefixed component names (such as Root, Input, Indicator) and index.barrel.ts with full names (SwitchRoot, CheckboxIndicator). The former was supposed to be used as import * as Foo from '@base_ui/react/Foo, while the latter import { FooRoot } from '@base_ui/react'.

Now we still have two index files per component, but only index.ts is meant to be imported. index.parts.ts aggregates and exports all parts of a component, while index.ts reexports them and names the exported namespace.

This makes all component parts accessible only through the component namespace:

import { Checkbox } from '@base_ui/react/Checkbox'; // or '@base_ui/react'
const CheckboxRoot = Checkbox.Root;

Closes #650

[mui-bot]

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[atomiks]

This still works with tree-shaking if you test a minimal Vite repo and build for production?

[michaldudak]

Yes, the resulting bundle does not contain the code of unused parts.