Improve handling of unmapped dependencies

Previously, all unmapped dependencies were treated equally. With this
commit a distinction is now made between unmapped direct dependencies
and unmapped transitive dependencies. By default, the build will fail
in the event of one or more unmapped direct dependencies but unmapped
transitive dependencies have no effect on the build's outcome. This
behaviour can be overridden via the  springioPlatform extension's
failOnUnmappedDirectDependency and failOnUnmappedTransitiveDependency
properties respectively. For example, to only fail on unmapped
transitive dependencies:

springioPlatform {
	failOnUnmappedDirectDependency = false
	failOnUnmappedTransitiveDependency = true
}