Modernize for module support and proper CLI library referencing

[jbuchbinder]

Modernize library to make use of go modules and use the new location of the referenced CLI library.

[dhogborg]

Thank you for you contribution!

What is the rationale behind having multiple go.mod files? This app does not publish multiple packages (in fact, non at all, since they reside in internal), and besides what would be better served by a go.work file.

I'm all for adding a go.mod file, but it should be a single one in the project root. If there is a demand for accessing the implementation as modules then renaming internal to pkg would be preferable to multiple go.mod files.