Fix local macro packages failing when SwiftSyntax prebuilts are enabled

Projects that depend on local macro packages no longer have those packages forced to consume incompatible SwiftSyntax prebuilt binaries. Tuist now keeps local external packages on source dependencies when prebuilt metadata is present, so remote packages can still use SwiftSyntax prebuilts while local macro targets build SwiftSyntax with the active toolchain. This resolves errors like importing SwiftCompilerPlugin from an incompatible .swiftmodule under .build/prebuilts. Also includes the related swifterpm update to ensure registry package resolution and prebuilt path handling work correctly.