There isn’t any such restriction. I would recommend getting debug logs (sonar.verbose=true) and checking the output: you’ll get a precise view of which files are seen/considered/indexed.
In fact @weijian_ni you are right, vendor directory is excluded by default, because we expect that you don’t want to analyze dependencies of your project (they should be analyzed separately in their own project).
You can change that behavior by un-setting sonar.go.exclusions property, just set it to empty value. I am going to update our documentation about this.