This is a first step in reversing the dependency flow around the `theme`
package in MDC. In a future commit, I'll make theme contain all the various
MaterialComponents theme definitions, and it will thus depend on the various
packages containing our components (widget, button, toggle, etc.).
The theme package will be empty as of this commit, but since I plan to use it
in the next one I've left the build infrastructure in place.
PiperOrigin-RevId: 180737806