Fix issue 1602 - Synced folders: includes silently ignored + no deduplication across targets

[4brunu]

Fixes #1602 — two bugs affecting syncedFolder source type:

• includes were silently ignored: When a TargetSource with type: syncedFolder specified includes, they
  had no effect. The fix computes exception paths by walking the directory tree and adding any
  files/folders not matched by the includes patterns as exceptions on the
  PBXFileSystemSynchronizedRootGroup.
• No deduplication across targets: When the same synced folder path was referenced by multiple
  targets, a new PBXFileSystemSynchronizedRootGroup object was created each time, resulting in duplicate
  entries. The fix caches created groups by path in syncedGroupsByPath and reuses them, merging
  explicitFolders from all referencing targets.

Changes

• SourceGenerator.swift: Added syncedGroupsByPath cache to deduplicate synced root groups across
  targets. Added syncedFolderExceptions(for:at:) to compute exception paths from includes/excludes
  patterns.
• PBXProjGenerator.swift: Minor supporting change.
• SourceGeneratorTests.swift: Added tests covering both fixes.

@AlexNsbmr can you please check if this PR fixes your issue?

Thanks

[4brunu]

@yonaskolb can you please help to review and merge those fixes to syncedFolder? Thanks

[yonaskolb]

Looks good, thanks @4brunu!