New license page. (#57588)

This is a PR addressing #57226 - Proposal: New UI for Licenses Page.

This PR replaces the previous single panel license page with one that uses a master/detail flow (MDFlow) to display packages and their respective licenses.

The License Page API remains unchanged. The logic for processing the license data is kept largely the same. This PR changes how the licenses are displayed, by introducing a responsive UI using the master/detail UI pattern. For now I am calling it Master Detail Flow, or MDFlow.

MDFlow manifests as two layouts depending on the screen size. On small and medium displays, as determined by the breakpoints given by the Material Design Spec, MDFlow utilises a nested layout. On large displays, MDFlow uses a two panel (lateral) layout. MDFlow is implemented in this PR using a Navigator for the nested layout, and a Stack for the lateral layout. The master and detail views are built using builders. For the interactive component, detail pages are requested from the master view using a proxy obtained by a widget lookup on the build context; MasterDetailFlow.of(context).