fix: use absolute path for iOS when user sourceDir is set

[alfonsocj]

Summary:

When setting iOS sourceDir in react-native.config.js, it doesn't convert the relative path to absolute as it does for Android.

Not having an absolute sourceDir could cause an issue similar to the one below:

[!] Invalid `Podfile` file: undefined method `[]' for nil:NilClass.

 #  from /Users/user/src/react-native/src/packages/app/ios/Podfile:27
 #  -------------------------------------------
 #  target 'app' do
 >    config = use_native_modules!
 #
 #  -------------------------------------------

When userConfig.sourceDir is not set, it uses the absolute project path which makes sense.

Test Plan:

I tested by building and creating an alias in node_modules for @react-native-community/cli-platform-ios on my current react native app and running yarn react-native config.

[thymikee]

Sounds about right, thanks! Mind adding a test to the packages/cli-config/src/__tests__/index-test.ts?

[alfonsocj]

@thymikee test added

[thymikee]

Thanks for the test! <3