Fix localized variant groups across targets and support folder resources

[eschwieb]

What was wrong

XcodeGen reused localized variant groups and their child file references across targets. That works only when every target uses the same localizations. If two targets selected different languages, generating the second target could move shared references out of the first target’s variant group or leave a same-named variant group detached from the project hierarchy.

XcodeGen also did not handle directories inside .lproj folders as localized folder resources. A structure such as:

Resources/
├── Base.lproj/
│   └── Help/
├── de.lproj/
│   └── Help/
└── en.lproj/
    └── Help/

needs to become a Help variant group whose language children are folder references. Previously, the children had no folder type and the variant group was not added to the resources phase, so Xcode would not copy the localized directories correctly.

What changed

Variant groups are now kept separate for each target, and localized child references are scoped to the variant group that owns them. Ordinary file references remain shared across the project.

Localized directories are now emitted as folder references and their variant groups are added to the resources phase. A non-base localization can create the folder variant group when the folder is missing from Base.lproj or when a target selects only another language.

Folder matching is also kept separate from extension-insensitive file matching, so resources such as Help/ and Help.strings do not accidentally end up in the same variant group.

Tests

The new tests cover:

• Different and identical language selections across targets
• Localized references used across multiple source entries
• Direct and localized uses of the same physical file
• Storyboard and strings companion resources
• Project serialization and reloading
• Localized folders copied as opaque folder references
• Localized folders alongside compiled XIB resources
• Targets selecting only a non-development language
• Folders missing from the base localization
• Same-stem folder and file collisions
• Parent ownership and build-phase membership

The complete Swift test suite passes.

Developed with assistance from GitHub Copilot. I did human review of each step along the way, and verified this addresses the issues we found in downstream code.