Uncaught exceptions in an overridden add_dependencies() method of a plugin
can prevent a stack being loaded from the database. To protect against
programming errors that can result in uncaught exceptions, separate the
calculation of implicit dependencies out from the calculation of explicit
dependencies, and ignore exceptions in the latter when dealing with an
existing stack.
Change-Id: I939dba57eeba5710bb77a1b30a872fca5d38ad71
Closes-Bug: #1554625