Plugin import scanner isn't passing in the library directory (#6323)

The Workspace type has a facility for scanning package plugins for imports.  But since it's not passing in the actual plugin directory provided by the caller (whether it's SwiftPM or Xcode), any modules provided by an IDE aren't picked up if they are guarded by `#if canImport()`.  This includes `XcodeProjectPlugin`, which is one of the additional plugin modules provided by Xcode.  The same would be true for other IDEs.

Changes:
- `Workspace.loadPluginImports` now also passes the path of the plugin directory to the import scanning
- extended the `testScanImportsInPluginTargets` unit test to check for this case

rdar://106387043

Author: abertelrud

Sources/Workspace/Workspace.swift

@@ -1293,7 +1293,7 @@ extension Workspace {
         packageGraph: PackageGraph,
         completion: @escaping(Result<[PackageIdentity: [String: [String]]], Error>) -> Void) {
         let pluginTargets = packageGraph.allTargets.filter{$0.type == .plugin}
-        let scanner = SwiftcImportScanner(swiftCompilerEnvironment: hostToolchain.swiftCompilerEnvironment, swiftCompilerFlags: hostToolchain.swiftCompilerFlags, swiftCompilerPath: hostToolchain.swiftCompilerPath)
+        let scanner = SwiftcImportScanner(swiftCompilerEnvironment: hostToolchain.swiftCompilerEnvironment, swiftCompilerFlags: hostToolchain.swiftCompilerFlags + ["-I", hostToolchain.swiftPMLibrariesLocation.pluginLibraryPath.pathString], swiftCompilerPath: hostToolchain.swiftCompilerPath)
         var importList = [PackageIdentity: [String: [String]]]()
 
         for pluginTarget in pluginTargets {

Tests/SPMBuildCoreTests/PluginInvocationTests.swift

@@ -1011,19 +1011,30 @@ class PluginInvocationTests: XCTestCase {
             try localFileSystem.writeFileContents(qPluginTargetDir.appending("plugin.swift"), string: """
                   import PackagePlugin
                   import XcodeProjectPlugin
+                  #if canImport(ModuleFoundViaExtraSearchPaths)
+                  import ModuleFoundViaExtraSearchPaths
+                  #endif
                   @main struct QBuildToolPlugin: BuildToolPlugin {
                       func createBuildCommands(
                           context: PluginContext,
                           target: Target
                       ) throws -> [Command] { }
                   }
                   """)
+            
+            // Create something that looks like another module that can be detected via `canImport()`.
+            let fakeExtraModulesDir = tmpPath.appending("ExtraModules")
+            try localFileSystem.createDirectory(fakeExtraModulesDir, recursive: true)
+            let fakeExtraModuleFile = fakeExtraModulesDir.appending("ModuleFoundViaExtraSearchPaths.swiftmodule")
+            try localFileSystem.writeFileContents(fakeExtraModuleFile, string: "")
+            
             /////////
             // Load a workspace from the package.
             let observability = ObservabilitySystem.makeForTesting()
             let workspace = try Workspace(
                 fileSystem: localFileSystem,
-                forRootPackage: packageDir,
+                location: try Workspace.Location(forRootPackage: packageDir, fileSystem: localFileSystem),
+                customHostToolchain: UserToolchain(destination: .hostDestination(), customLibrariesLocation: .init(manifestLibraryPath: fakeExtraModulesDir, pluginLibraryPath: fakeExtraModulesDir)),
                 customManifestLoader: ManifestLoader(toolchain: UserToolchain.default),
                 delegate: MockWorkspaceDelegate()
             )
@@ -1060,8 +1071,8 @@ class PluginInvocationTests: XCTestCase {
                         } else if pkg.description == "otherpackage" {
                             XCTAssertNotNil(dict[pkg]?["QPlugin"])
 
-                            let possibleImports1 = ["PackagePlugin", "XcodeProjectPlugin"]
-                            let possibleImports2 = ["PackagePlugin", "XcodeProjectPlugin", "_SwiftConcurrencyShims"]
+                            let possibleImports1 = ["PackagePlugin", "XcodeProjectPlugin", "ModuleFoundViaExtraSearchPaths"]
+                            let possibleImports2 = ["PackagePlugin", "XcodeProjectPlugin", "ModuleFoundViaExtraSearchPaths", "_SwiftConcurrencyShims"]
                             XCTAssertTrue(entry["QPlugin"] == possibleImports1 ||
                                           entry["QPlugin"] == possibleImports2)
                             count += 1