I want to add some more functionality on this application, which won't
really make sense when it has a single entry point. So update it so that
new functionality can be implemented in a sub command.

A large number of tests were moved under the `main` module. To ensure
the coverage of other modules is still included when testing this module
add the `-coverpkg` command to `go test` (see docs[1])

[1] https://pkg.go.dev/cmd/go#hdr-Testing_flags