fix(@ngtools/webpack): don't delete virtual files for resources still on disk

Fix angular#15453

Author: filipesilva

src/compiler_host.ts

@@ -141,10 +141,16 @@ export class WebpackCompilerHost implements ts.CompilerHost {
       return;
     }
 
-    for (const ext of this._virtualStyleFileExtensions) {
-      const virtualFile = (fullPath + ext) as Path;
-      if (this._memoryHost.exists(virtualFile)) {
-        this._memoryHost.delete(virtualFile);
+    if (!exists) {
+      // At this point we're only looking at resource files (html/css/scss/etc).
+      // If the original was deleted, we should delete the virtual files too.
+      // If the original it wasn't deleted we should leave them to be overwritten, because webpack
+      // might begin the loading process before our plugin has re-emitted them.
+      for (const ext of this._virtualStyleFileExtensions) {
+        const virtualFile = (fullPath + ext) as Path;
+        if (this._memoryHost.exists(virtualFile)) {
+          this._memoryHost.delete(virtualFile);
+        }
       }
     }
   }