Better data type for Linkage Errors and Artifact Problems?

@elharo says:

> I still think we're missing an abstraction here. We have problems of which there are several subtypes including missing artifacts and linkage errors. The linkage errors we don't have a class for. Instead we're storing them as an entry in the multimap.

https://github.com/GoogleCloudPlatform/cloud-opensource-java/pull/1135/files/a2dc1f7d9cd2ff0950b2c4d01e5b80c2018d52f5#r367607402

As of now
- Linkage errors are represented as `Multimap`
Many-to-many relationship between SymbolProblem and ClassFile.
- Artifact Problems are represented as `ArtifactProblem`
Example: non-resolvable artifacts in a dependency tree, when constructing a class path.