fix(@ngtools/webpack): do not run ngcc when node_modules does not exist

Prior to this change the CLI would fail with an error when ngcc could not
be run due to a missing `node_modules` directory, as is the case with Yarn
Plug'n'Play for example. The workaround was to create an empty `node_modules`
directory, but this is inconvenient and shouldn't make any difference.

This commit removes the error and simply skips ngcc processing.

Author: JoostK

packages/ngtools/webpack/src/ngcc_processor.ts

@@ -32,7 +32,7 @@ type ResolverWithOptions = ReturnType<Compiler['resolverFactory']['get']>;
 export class NgccProcessor {
   private _processedModules = new Set<string>();
   private _logger: NgccLogger;
-  private _nodeModulesDirectory: string;
+  private _nodeModulesDirectory: string | null;
 
   constructor(
     private readonly compilerNgcc: typeof import('@angular/compiler-cli/ngcc'),
@@ -54,8 +54,10 @@ export class NgccProcessor {
 
   /** Process the entire node modules tree. */
   process() {
-    // Under Bazel when running in sandbox mode parts of the filesystem is read-only.
-    if (process.env.BAZEL_TARGET) {
+    // Under Bazel when running in sandbox mode parts of the filesystem is read-only, or when using
+    // Yarn PnP there may not be a node_modules directory. ngcc can't run in those cases, so the
+    // processing is skipped.
+    if (process.env.BAZEL_TARGET || !this._nodeModulesDirectory) {
       return;
     }
 
@@ -162,18 +164,20 @@ export class NgccProcessor {
     }
   }
 
-  /** Process a module and it's depedencies. */
+  /** Process a module and its dependencies. */
   processModule(
     moduleName: string,
     resolvedModule: ts.ResolvedModule | ts.ResolvedTypeReferenceDirective,
   ): void {
     const resolvedFileName = resolvedModule.resolvedFileName;
     if (
+      !this._nodeModulesDirectory ||
       !resolvedFileName ||
       moduleName.startsWith('.') ||
       this._processedModules.has(resolvedFileName)
     ) {
-      // Skip when module is unknown, relative or NGCC compiler is not found or already processed.
+      // Skip when module_modules directory is not present, module is unknown, relative or the
+      // NGCC compiler is not found or already processed.
       return;
     }
 
@@ -232,7 +236,7 @@ export class NgccProcessor {
     }
   }
 
-  private findNodeModulesDirectory(startPoint: string): string {
+  private findNodeModulesDirectory(startPoint: string): string | null {
     let current = startPoint;
     while (path.dirname(current) !== current) {
       const nodePath = path.join(current, 'node_modules');
@@ -243,7 +247,7 @@ export class NgccProcessor {
       current = path.dirname(current);
     }
 
-    throw new Error(`Cannot locate the 'node_modules' directory.`);
+    return null;
   }
 
   private findPackageManagerLockFile(projectBasePath: string): {