Override base theme library overrides

A theme generated with starterkit is based on stable9 and creates templates and libraries with some enhanced defaults, similar to the Classy theme.

Themes can override or extend other libraries.

When a sub theme tries to override a library that is already overridden by base theme, the base theme's file path must be used.

Instead of (css file defined by views_ui/admin.styling library):

library-override:
  views_ui/admin.styling:
    css:
      theme:
        css/views_ui.admin.theme.css

Use (css file defined by stable9 override):

library-override:
  views_ui/admin.styling:
    css:
      theme:
        /core/themes/stable9/css/views_ui/views_ui.admin.theme.css

It has to have the full file path from the docroot to work.