use absolute imports for vx components, update builds to support absolute imports for data-ui components

[williaster]

currently we import components from vx via import { MyComponent } from '@vx/package' which means bundles will include the entire vx package we are importing from. this could be improved with absolute imports import MyComponent from @vx/package/build/....

similarly we only build an index for each @data-ui package which means users get the entire package when they import a single component. this could be improved by exporting the entire build directory to support absolute imports for @data-ui components.

[williaster]

closed with tree shaking and per-file builds in #111