Correct test input code to support older go versions

[perrito666]

Samples used for testing need to have build tags for new and old style:

//go:build tag
// +build tag

The particular problem here, seems to be that 1.16 was actually importing the package referred in the TestGoModules test (import "golang.org/x/text/unicode/norm") which, in the CI system's version lacks an old style go tag comment. Hence go complaining of a //go:build without a // +build. I made all files bi-tagged for consistency.

[natefinch]

Thank you! Nothing should be imported outside the stdlib anyway. I don't want this tool to require outside imports when you build it.