Invoke hook after composer dependencies are built [D8]

Follow-up to #2272547: Invoke hook after composer dependencies are built

Proposal: introduce composer_manager_build_complete(), which fires after a composer_manager build is complete.

Use case:

• User enables module A, which has a library dependency managed by composer_manager
• Module A needs to fire off a few functions in hook_install(), but it cannot because dependencies are not yet build when that hook in invoked
• Instead, Module A implements composer_manager_build_complete(). which is fired after dependencies are built. Necessary install functions are then called within that implementation.