go.mod/go.sum should be on the module root

[ktr]

What

Same as the title.

Why

According to Using Go Modules, it says

The go.mod file defines the module’s module path, which is also the import path used for the root directory, and its dependency requirements, which are the other modules needed for a successful build.

So, the directory that go.mod has also should have Go files to succeeded in build.