feat: start sketching out the oonirun package (#842)

This diff refactors the ./internal/cmd/miniooni pkg and moves the code
for running experiments inside of the ./internal/oonirun pkg.

It's the first concrete step towards https://github.com/ooni/probe/issues/2184.

internal/cmd/miniooni/main.go

@@ -2,6 +2,10 @@
 // with a CLI interface similar to MK and OONI Probe v2.x.
 package main
 
+//
+// Main function
+//
+
 import (
 	"fmt"
 	"os"