Move cobertura from `main` to a sub-package.

Could the code be moved from `main` to a sub-package so it becomes importable?

Right now I am seeing
```
./build/.modcache/github.com/ryancurrah/gomodguard@v1.3.2/tools.go:5:8: import "github.com/t-yuki/gocover-cobertura" is a program, not an importable package
```

Thanks in advance!