On PR to submodule parent package was forked instead of submodule

[3y3]

• Extension version: v0.100.1
• VSCode Version: 1.9.3
• OS: MacOS
• Repository Clone Configuration (single repository/fork of an upstream repository): single repository with submodules
• Github Product (Github.com/Github Enterprise version x.x.x): Github.com

Steps to Reproduce:

tl;dr: On PR create for submodule, extension will fork main repository instead of submodule.
Looks like it's the same as
#4487
#5181

We found issue on diplodoc metapackage. But it is reproducible on any repo with submodules.

Prerequisites:
Metapackage - organisation owned repo with submodules and enabled codespaces.
Submodule - any organisation owned repo linked to metapackage.
Alice - user out of organization scope.

1. Create codespace for metapackage and open it in browser VSCode
2. Pull submodules. We use this small script to pull all submodules in one time, but looks like there is nothing special.
3. Make some change to submodule.
4. Create new branch in submodule. Commit changes (with help of Github Pull Request Extension)
5. Push the button "Create Pull Request"
6. Check autofilled fields (looks like at this step all is normal: BASE and MERGE fields refers to submodule branches)
7. Push "Create" button.
8. VSCode notifies Alice about restriction to pull directly to organization and proposes to create fork.
9. Submit to fork submodule
   9.1 Current behaviour: fork for metapackage was created, pr flow was broken on next step with reasonable error.
   
   9.2 Expected behaviour: fork for submodule was created, pr flow finished succesefully.
10. At second try to push "Create" button error message was changed to