Fix LSP re-indexing files that were already indexed

[rockbruno]

• Adds a catch-all prefix mapping "./" -> workspace root to the sourcekit-lsp config
• Add support for outputPathsProvider (returning to the LSP in advance where each index file will be)
• Disable WMO and global_index_store in the example project as these turned out to conflict with index files generation
• Fix watch app's extension not being propagated by the aspect builds
• Fix Swift compiler arguments being wrongly shared across platforms