refactor(miniooni): divide et impera (#912)

This diff splits miniooni's implementation in smaller and more
easily tractable blocks ahead of future refactoring.

I'm trying to make `miniooni oonirun -i URL` as possible as
`miniooni -i URL oonirun`, because users typically expect this
kind of flexibity from modern Unix commands.

Part of https://github.com/ooni/probe/issues/2184

internal/cmd/miniooni/main.go

@@ -1,11 +0,0 @@
-// Command miniooni is a simple binary for research and QA purposes
-// with a CLI interface similar to MK and OONI Probe v2.x.
-package main
-
-//
-// Main function
-//
-
-func main() {
-	Main()
-}