Addons: don't return current translation and include source language in projects.translations field

[humitos]

Currently, project.translations is returning all the translations from main_project. I realized that we don't always want this. This is only useful when browsing the source language of the documentation. However, when reading any of its translations project.translations should not show the current translation (the one the user is reading) and should show the source language as translation as well.

Example

If the project has English (source language), Spanish and Japanese translations, projects.translations should contain:

• Spanish and Japanese, when hitting the English translation
• English and Japanese, when hitting the Spanish translation

How to reproduce it

1. Go to https://cpython-previews.readthedocs.io/es/latest/
2. Open the flyout
3. You will see that es is duplicated
   

[humitos]

I think something like this could work:

diff --git a/readthedocs/proxito/views/hosting.py b/readthedocs/proxito/views/hosting.py
index 4b598e21b..780e09091 100644
--- a/readthedocs/proxito/views/hosting.py
+++ b/readthedocs/proxito/views/hosting.py
@@ -342,7 +342,12 @@ class AddonsResponse:
                 )
 
         main_project = project.main_language_project or project
-        project_translations = main_project.translations.all().order_by("language")
+        # Exclude the current project since we don't want to return itself as a translation
+        project_translations = main_project.translations.all().exclude(slug=project.slug).order_by("language")
+
+        # Include main project as translation if the current project is one of the translations
+        if project != main_project:
+            project_translations |= Project.objects.filter(slug=main_project.slug)
 
         data = {
             "api_version": "1",

[humitos]

I tested the previous diff locally and it works fine 👍🏼 . I need to open a PR and write some tests.