ws: Add cockpit_web_server_get_document_roots() function

This is a handy accessor to get the already resolved expanded
root document paths from the web server.
pkg: Shallow cockpit AMD module identifiers

These take away the 'cockpit/' prefix, in our module ids. It was
repetitious and made it hard for tests to run in our checkout tree.

If we're not the actual loader implementation, then leave it up
to the other AMD loader to map our module names (for the modules that
they are interested in).