Prepare for Go modules

[hajimehoshi]

This is just random notes to myself:

• go.mod is needed not only at ebiten package but also all the example main packages.
• To test a new major version on non-master branch, it looks like at least tagging is required. => branch-lookup would solve this: x/vgo: look up branches (not just tags) in version fixer golang/go#24045
• vgo run with specifying Go files doesn't recognize the main packages version-ed path. In other words, we can't have the below go.mod for vgo run?

module "github.com/hajimehoshi/ebiten/v2/examples/blocks"

=> (v)go run with package name would be available golang/go#22726

• vgo build accepts both modules paths (version-ed and not version-ed) for main packages
• Is it possible to prevent from mixing old and new versions?

[hajimehoshi]

As Go 1.10.3 already has minimal support for semantic versioning import paths, I think I can release 2.0 sooner than I expected (e.g. next to 1.8)?

[hajimehoshi]

go.mod already exists and let's keep updating it.