Add imported register source key to dependency diagram.

[rob-metalinkage]

It would be great to see the import dependencies colour coded according to register - this would combine with register governance to make it clear how stable all parts are.

Note that using the bblocks: $ref method means this will be automatically updated every build, even if things move between registers.

(how to handle a "ghost" moved to another register is a downstream problem - maybe we need to have a deprecated/moved mechanism?)

(Need to create a video tutorial on this aspect - its actually quite powerful)

[rob-metalinkage]

Completed - beautiful!