[Dependency Scanning] Do not specify placeholder modules by default as inputs to scanning actions

Instead allow the dependency scanning oracle to be configured such that the scanning actions use placeholders.

In contexts where the driver's client can guarantee that all module dependencies have been built prior to planning of the dependee, placeholders are not necessary in the scanner, becuase the binary modules will simply be discovered on the filesystem.

Resolves rdar://102339586

Author: artemcm

Sources/SwiftDriver/ExplicitModuleBuilds/InterModuleDependencies/InterModuleDependencyOracle.swift

@@ -30,7 +30,17 @@ import Dispatch
 /// An abstraction of a cache and query-engine of inter-module dependencies
 public class InterModuleDependencyOracle {
   /// Allow external clients to instantiate the oracle
-  public init() {}
+  /// - Parameter scannerRequiresPlaceholderModules: Configures this driver's/oracle's scanner invocations to
+  /// specify external module dependencies to be treated as placeholders. This is required in contexts
+  /// where the dependency scanning action is invoked for a module which depends on another module
+  /// that is part of the same build but has not yet been built. Treating it as a placeholder
+  /// will allow the scanning action to not fail when it fails to detect this dependency on
+  /// the filesystem. For example, SwiftPM plans all targets belonging to a package before *any* of them
+  /// are built. So this setting is meant to be used there. In contexts where planning a module
+  /// necessarily means all of its dependencies have already been built this is not necessary.
+  public init(scannerRequiresPlaceholderModules: Bool = false) {
+    self.scannerRequiresPlaceholderModules = scannerRequiresPlaceholderModules
+  }
 
   @_spi(Testing) public func getDependencies(workingDirectory: AbsolutePath,
                                              moduleAliases: [String: String]? = nil,
@@ -145,5 +155,7 @@ public class InterModuleDependencyOracle {
 
   /// A reference to an instance of the compiler's libSwiftScan shared library
   private var swiftScanLibInstance: SwiftScan? = nil
+
+  internal let scannerRequiresPlaceholderModules: Bool
 }
 

Sources/SwiftDriver/ExplicitModuleBuilds/ModuleDependencyScanning.swift

@@ -74,7 +74,8 @@ public extension Driver {
     // FIXME: MSVC runtime flags
 
     // Pass in external target dependencies to be treated as placeholder dependencies by the scanner
-    if let externalTargetDetailsMap = externalTargetModuleDetailsMap {
+    if let externalTargetDetailsMap = externalTargetModuleDetailsMap,
+       interModuleDependencyOracle.scannerRequiresPlaceholderModules {
       let dependencyPlaceholderMapFile =
       try serializeExternalDependencyArtifacts(externalTargetDependencyDetails: externalTargetDetailsMap)
       commandLine.appendFlag("-placeholder-dependency-module-map-file")