Make `go tool minimock` work

Hi,

I haven't checked, but I hope it's doable without tons of changes. This makes maintaining tool dependencies so much easier.

```
go get -tool github.com/gojuno/minimock/v3@latest
go tool minimock ...
```

Above should use exact version for everybody no matter what's installed in the GOBIN dir. More on this https://tip.golang.org/doc/modules/managing-dependencies#tools

At the moment above fails with the following error:

```
go tool minimock -i VersionService -o ./mocks -s _mock.go -g
package github.com/gojuno/minimock/v3 is not a main package
```

Sure I still use `go get -tool github.com/gojuno/minimock/v3@latest`, but before `go generate` I do `go install tool` to get exact version in the GOBIN.