ignore unknown components in dependencies of PlantUML diagrams

[tfij]

The current implementation prevents using PlantUML diagrams as rules that are used for wider documentation purposes (e.g. containing a database component or some other dependency that's irrelevant from the point of an ArchUnit test). The consequence of ignoring such unknown components is only to allow fewer dependencies and fewer classes to be present. So, at best the test will fail as a false positive, if the components are declared wrongly by accident. Thus, there is no major harm in simply ignoring these unknown components and giving users a little more freedom what elements they want to use in their diagrams.

Resolves: #1132

[tfij]

@codecholeric thanks for CR all comments applied