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/utils.go

@@ -0,0 +1,88 @@
+package main
+
+//
+// Utility functions
+//
+
+import (
+	"errors"
+	"net/url"
+	"os"
+	"runtime"
+	"strings"
+
+	"github.com/ooni/probe-cli/v3/internal/runtimex"
+)
+
+// regularFileExists returns true if the given filepath exists and is a regular file
+func regularFileExists(filepath string) bool {
+	stat, err := os.Stat(filepath)
+	return err == nil && stat.Mode().IsRegular()
+}
+
+// splitPair takes in input a string in the form KEY=VALUE and splits it. This
+// function returns an error if it cannot find the = character to split the string.
+func splitPair(s string) (string, string, error) {
+	v := strings.SplitN(s, "=", 2)
+	if len(v) != 2 {
+		return "", "", errors.New("invalid key-value pair")
+	}
+	return v[0], v[1], nil
+}
+
+// mustMakeMapStringString makes a map from string to string using as input a list
+// of key-value pairs used to initialize the map, or panics on error
+func mustMakeMapStringString(input []string) (output map[string]string) {
+	output = make(map[string]string)
+	for _, opt := range input {
+		key, value, err := splitPair(opt)
+		runtimex.PanicOnError(err, "cannot split key-value pair")
+		output[key] = value
+	}
+	return
+}
+
+// mustMakeMapStringAny makes a map from string to any using as input a list
+// of key-value pairs used to initialize the map, or panics on error
+func mustMakeMapStringAny(input []string) (output map[string]any) {
+	output = make(map[string]any)
+	for _, opt := range input {
+		key, value, err := splitPair(opt)
+		runtimex.PanicOnError(err, "cannot split key-value pair")
+		output[key] = value
+	}
+	return
+}
+
+// mustParseURL parses the given URL or panics
+func mustParseURL(URL string) *url.URL {
+	rv, err := url.Parse(URL)
+	runtimex.PanicOnError(err, "cannot parse URL")
+	return rv
+}
+
+// gethomedir returns the home directory. If optionsHome is set, then we
+// return that string as the home directory. Otherwise, we use typical
+// platform-specific environment variables to determine the home. In case
+// of failure to determine the home dir, we return an empty string.
+func gethomedir(optionsHome string) string {
+	// See https://gist.github.com/miguelmota/f30a04a6d64bd52d7ab59ea8d95e54da
+	if optionsHome != "" {
+		return optionsHome
+	}
+	if runtime.GOOS == "windows" {
+		home := os.Getenv("HOMEDRIVE") + os.Getenv("HOMEPATH")
+		if home == "" {
+			home = os.Getenv("USERPROFILE")
+		}
+		return home
+	}
+	if runtime.GOOS == "linux" {
+		home := os.Getenv("XDG_CONFIG_HOME")
+		if home != "" {
+			return home
+		}
+		// fallthrough
+	}
+	return os.Getenv("HOME")
+}