[Explicit Module Builds] Point module dependency modules to be stored in the module cache

Preprocess the dependency graph, setting module paths to be relative to the module cache path, if one is present.
Only do so for modules that do not already provide a path.

Author: artemcm

Sources/SwiftDriver/Explicit Module Builds/ExplicitModuleBuildHandler.swift

@@ -175,6 +175,10 @@ public typealias ExternalDependencyArtifactMap =
       }
     }
 
+    // Set the output path
+    commandLine.appendFlag(.o)
+    commandLine.appendPath(try VirtualPath(path: moduleInfo.modulePath))
+
     swiftModuleBuildCache[moduleId] = Job(
       moduleName: moduleId.moduleName,
       kind: .emitModule,

Sources/SwiftDriver/Jobs/Planning.swift

@@ -262,11 +262,41 @@ extension Driver {
       try dependencyGraph.resolvePlaceholderDependencies(using: externalDependencyArtifactMap!)
     }
 
-    // Re-scan Clang modules at all the targets they will be built against
+    // Re-scan Clang modules at all the targets they will be built against.
     try resolveVersionedClangDependencies(dependencyGraph: &dependencyGraph)
+
+    // Set dependency modules' paths to be saved in the module cache.
+    try updateDependencyModulesWithModuleCachePath(dependencyGraph: &dependencyGraph)
+
     return dependencyGraph
   }
 
+  /// Update the given inter-module dependency graph to set module paths to be within the module cache,
+  /// if one is present.
+  private mutating func updateDependencyModulesWithModuleCachePath(dependencyGraph:
+                                                                    inout InterModuleDependencyGraph)
+  throws {
+    let moduleCachePath = parsedOptions.getLastArgument(.moduleCachePath)?.asSingle
+    if moduleCachePath != nil {
+      for (moduleId, moduleInfo) in dependencyGraph.modules {
+        // Output path on the main module is determined by the invocation arguments.
+        guard moduleId.moduleName != dependencyGraph.mainModuleName else {
+          continue
+        }
+        let modulePath = moduleInfo.modulePath
+        // Only update paths on modules which do not already specify a path beyond their module name
+        // and a file extension.
+        if modulePath == moduleId.moduleName + ".swiftmodule" ||
+            modulePath == moduleId.moduleName + ".pcm" {
+          // Use VirtualPath to get the OS-specific path separators right.
+          let modulePathInCache =
+            try VirtualPath(path: moduleCachePath!).appending(component: modulePath).description
+          dependencyGraph.modules[moduleId]!.modulePath = modulePathInCache
+        }
+      }
+    }
+  }
+
   /// Create a job if needed for simple requests that can be immediately
   /// forwarded to the frontend.
   public mutating func immediateForwardingJob() throws -> Job? {