java: build a package dependency graph #230

Dependency analysis covers Go only. For Java, the module graph has the packages of the tree as nodes. The package of a file comes from its package declaration. A single-type, on-demand or static import of a package of the tree adds an edge to that package. An import from the JDK or a library adds no edge. Abstractness is the share of the public types of a package that are interfaces or abstract classes.