Check if go.mod exists while init

[soeti]

If for something reason we miss go.mod and then run "go run github.com/99designs/gqlgen init" we get cascade errors like:

projects/source/go/test/typeparam/issue51836.dir imports
        ./aa: "./aa" is relative, but relative import paths are not supported in module mode
tidy failed: go mod tidy failed: exit status 

This simple check prevent this

[coveralls]

Coverage decreased (-0.2%) to 75.409% when pulling 92b2d8d on soeti:gomod-miss-check into 89e91da on 99designs:master.

[StevenACoffman]

Thanks!