Update to use Go modules

[thisisaaronland]

• Replaces gopkg stuff with go mod stuff since the latter is the default/built-in dependency system as of 1.12
• Add a Makefile for building tools in local bin directory (which is excluded from commits)
• Update .gitignore file to exclude Emacs ~ files

Personally I have no feelings about package/dependency managers beyond "If Go is going to settle this for everyone, as with go fmt then I am okay with it".

Go modules depend on things having v{X}.{Y}.{Z} semantic tags/releases so this package should probably start minting releases too, in the interest of playing nice with others.

[nvkelso]

@zerebubuth can you have a look at this PR since Ian is away?

[iandees]

I started this using dep, which I thought was the preferred way of doing dependencies with Go nowadays.

[zerebubuth]

Yeah, I thought that dep was the current / future preferred way too. But looking at their README:

dep was the "official experiment." The Go toolchain, as of 1.11, has adopted an approach that sharply diverges from dep.

So... I guess that means that this week the official preferred way is go mod? What do you think, @iandees?

[iandees]

Sounds good to me. Guess I should read more about go modules then :)

[zerebubuth]

Great, thanks!