all: rename module and move main pacakge to cmd/asdf

[DeedleFake]

Fixes #84.

This also fixes a few other minor things, but in so doing causes go test to fail because a test was not being run at all before due to being named incorrectly is now running and it fails.

[DeedleFake]

I should note that go install will include the version of the main module into the metadata in the executable automatically. It can be gotten with debug.ReadBuildInfo(). go build doesn't currently include that, but that's being worked on.

[Stratus3D]

This repo (github.com/asdf-vm/asdf-core-go) is temporary and will be archived in a couple weeks. All the changes in this repo will soon be merged into github.com/asdf-vm/asdf once most of the test code in main_test.go is passing.

Should we make this name github.com/asdf-vm/asdf? What's the difference between github.com/asdf-vm/asdf and asdf?

[DeedleFake]

The module name should match the import path that identifies the repo. If the plan is to move the code into another repo, then it should match the repo it's moved to. It can be renamed now or when it's moved. Alternatively, you can use a vanity URL, i.e. asdf-vm.com. Then people could, for example, use go install asdf-vm.com/cmd/asdf@latest to install the command. That would allow you to move repos without having to rename the module. I use a vanity URL, deedles.dev, for my personal projects.

[Stratus3D]

Let's go with github.com/asdf-vm/asdf since that is where this code will live in a few weeks. I don't expect many people to use this asdf-core-go repo, and if they do they'll hopefully see it's not ready for regular use. I've updated the repo description to make this clear.

[Stratus3D]

Thanks for the PR @DeedleFake. I left a few comments. Looks like PR checks failed, I will investigate at a later time. Otherwise looks like something I can merge.