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.

先從 issue 討論和所引用的 pull request diff 開始，然後將目前的 Multimap 表示與 ArtifactProblem 進行比較。Done 應確立一個明確選擇的抽象，同時涵蓋 linkage error 和 artifact problem，並明確說明它們各自情況之間的區別。

由索引模型根據 Issue 內容生成。