GoMod: Missing (transitive) dependencies of dependencies without a 'go.mod'

[fviernau]

GoMod.kt constructs the dependency tree based on the module graph obtained via go mod graph.
It turned out that the module graph only contains edges defined in any go.mod file of any transitive dependency [1]. So, there are no edges for the dependencies of any module without a go.mod file.

The problem is reproduced by a functional test added by [2].

[1] golang/go#50656 (comment)
[2] #10102