Make language mappings deterministic

Consider this scenario: Someone has in their config file:

```ini
[main]
lang_map = it: it-IT

[RESOURCE_ID]
lang_map = it: it_IT
```

Before we created a local-to-remote map that looked like this:

```json
{"it-IT": "it", "it_IT", "it"}
```

And then, to create the remote-to-local map, we reversed it. The problem
was that, since both values are the same, the key that ended up being
selected was random, so it could either be:

```json
{"it": "it-IT"}
```

or

```json
{"it": "it_IT"}
```

To fix the issue, we now create remote-to-local first, going over the
global configuration first and the resource-level configuration second
so that the resource-level will prevail and thus take preference, and
then we create the local-to-remote by reversing. So,

First, we will consume the global configuration:

```json
{"it": "it-IT"}
```

Then, as the resource-level configuration is consumed, it will replace
the old key:

```json
{"it": "it_IT"}
```

And then, the local-to-remote will end up being:

```json
{"it_IT": "it"}
```

This is the deterministic and the intended behaviour, since
resource-level configuration should take precedence.

internal/txlib/pull.go

@@ -169,13 +169,11 @@ func (task *ResourcePullTask) Run(send func(string), abort func()) {
 	}
 	sendMessage("Getting info", false)
 
-	localToRemoteLanguageMappings := makeLocalToRemoteLanguageMappings(
+	remoteToLocalLanguageMappings := makeRemoteToLocalLanguageMappings(
 		*cfg,
 		*cfgResource,
 	)
-	remoteToLocalLanguageMappings := makeRemoteToLocalLanguageMappings(
-		localToRemoteLanguageMappings,
-	)
+	localToRemoteLanguageMappings := reverseMap(remoteToLocalLanguageMappings)
 
 	var err error
 	var resource *jsonapi.Resource

internal/txlib/push.go

@@ -493,10 +493,12 @@ func (task *ResourcePushTask) Run(send func(string), abort func()) {
 		}
 	}
 	if args.Translation { // -t flag is set
-		localToRemoteLanguageMappings := makeLocalToRemoteLanguageMappings(
-			*cfg,
-			*cfgResource,
-		)
+		localToRemoteLanguageMappings := reverseMap(
+            makeRemoteToLocalLanguageMappings(
+                *cfg,
+                *cfgResource,
+            ),
+        )
 		overrides := cfgResource.Overrides
 
 		sendMessage("Fetching remote languages", false)

internal/txlib/utils.go

@@ -124,7 +124,7 @@ func stringSliceContains(haystack []string, needle string) bool {
 	return false
 }
 
-func makeLocalToRemoteLanguageMappings(
+func makeRemoteToLocalLanguageMappings(
 	cfg config.Config, cfgResource config.Resource,
 ) map[string]string {
 	// In the configuration, the language mappings are "remote code -> local
@@ -133,21 +133,19 @@ func makeLocalToRemoteLanguageMappings(
 	// reverse the maps
 
 	result := make(map[string]string)
-	for key, value := range cfg.Local.LanguageMappings {
-		result[value] = key
+	for transifexLanguageCode, localLanguageCode := range cfg.Local.LanguageMappings {
+		result[transifexLanguageCode] = localLanguageCode
 	}
-	for key, value := range cfgResource.LanguageMappings {
+	for transifexLanguageCode, localLanguageCode := range cfgResource.LanguageMappings {
 		// Resource language mappings overwrite "global" language mappings
-		result[value] = key
+		result[transifexLanguageCode] = localLanguageCode
 	}
 	return result
 }
 
-func makeRemoteToLocalLanguageMappings(
-	localToRemoteLanguageMappings map[string]string,
-) map[string]string {
+func reverseMap(remoteToLocalLanguageMappings map[string]string) map[string]string {
 	result := make(map[string]string)
-	for key, value := range localToRemoteLanguageMappings {
+	for key, value := range remoteToLocalLanguageMappings {
 		result[value] = key
 	}
 	return result