[Explicit Module Builds] Add another shim for backwards-compatibility with API currently used by SwiftPM to communicate external build artifacts

Author: artemcm

Sources/SwiftDriver/Driver/Driver.swift

@@ -320,7 +320,10 @@ public struct Driver {
     diagnosticsEngine: DiagnosticsEngine = DiagnosticsEngine(handlers: [Driver.stderrDiagnosticsHandler]),
     fileSystem: FileSystem = localFileSystem,
     executor: DriverExecutor,
+    // FIXME: Duplication with externalBuildArtifacts and externalTargetModulePathMap
+    // is a temporary backwards-compatibility shim to help transition SwiftPM to the new API
     externalBuildArtifacts: ExternalBuildArtifacts? = nil,
+    externalTargetModulePathMap: ExternalTargetModulePathMap? = nil,
     interModuleDependencyOracle: InterModuleDependencyOracle? = nil
   ) throws {
     self.env = env
@@ -329,7 +332,11 @@ public struct Driver {
     self.diagnosticEngine = diagnosticsEngine
     self.executor = executor
 
-    self.externalBuildArtifacts = externalBuildArtifacts
+    if let externalArtifacts = externalBuildArtifacts {
+      self.externalBuildArtifacts = externalArtifacts
+    } else if let externalTargetPaths = externalTargetModulePathMap {
+      self.externalBuildArtifacts = (externalTargetPaths, [:])
+    }
 
     if case .subcommand = try Self.invocationRunMode(forArgs: args).mode {
       throw Error.subcommandPassedToDriver

Sources/SwiftDriver/Explicit Module Builds/ExplicitDependencyBuildPlanner.swift

@@ -15,6 +15,9 @@ import Foundation
 
 /// A map from a module identifier to a path to its .swiftmodule file.
 public typealias ExternalTargetModulePathMap = [ModuleDependencyId: AbsolutePath]
+
+// FIXME: ExternalBuildArtifacts is a temporary backwards-compatibility shim
+// to help transition SwiftPM to the new API.
 /// A tuple all external artifacts a build system may pass in as input to the explicit build of the current module
 /// Consists of a map of externally-built targets, and a map of all previously discovered/scanned modules.
 public typealias ExternalBuildArtifacts = (ExternalTargetModulePathMap, ModuleInfoMap)