Angular 9 CLI ignores custom .d.ts files

I have to maintain an old Angular 9 App. I cannot upgrade to Angular 10 or higher.

Consider the following import:

import Keycloak, { KeycloakProfile } from "keycloak-js";


The typescript compiler recognizes that import without any errors, because this is my respective type augmentation file under src/typing/keycloak-js.d.ts. VsCode even allows my to go to the respective typings, so the TS compiler recognized my custom .d.ts file.

declare module "keycloak-js" {
import Keycloak from "keycloak-js/lib/keycloak";
export default Keycloak;
export * from "keycloak-js/lib/keycloak";
}


In my package.json:

"dependencies": {
"keycloak-js": "^26.1.4",
}


This is my tsconfig.json:

{
"compileOnSave": false,
"compilerOptions": {
"allowSyntheticDefaultImports": true,
"allowJs": true,
"baseUrl": "./",
"outDir": "./dist/out-tsc",
"sourceMap": true,
"declaration": false,
"downlevelIteration": true,
"esModuleInterop": true,
"experimentalDecorators": true,
"module": "es2020",
"moduleResolution": "node",
"importHelpers": true,
"target": "es2015",
"resolveJsonModule": true,
"typeRoots": ["node_modules/@types", "./src/typing"],
"lib": ["es2017", "esnext.array", "dom"],
"paths": {
"@app/*": ["./src/app/*"],
"@environment/*": ["./src/environments/*"],
"keycloak-js": ["./src/typing/keycloak-js"]
}
},
"angularCompilerOptions": {
"fullTemplateTypeCheck": true,
"strictInjectionParameters": true,
"enableIvy": false
}
}


And this is my tsconfig.app.json:

{
"extends": "./tsconfig.json",
"compilerOptions": {
"outDir": "./out-tsc/app",
"types": []
},
"files": ["src/main.ts", "src/polyfills.ts"],
"include": ["src/**/*.ts", "src/typing/*.d.ts"],
"exclude": ["src/test.ts", "src/**/*.spec.ts"]
}


But the Angular CLI notes the following error when starting my Angular 9 app via ng serve:

ERROR in ./src/app/utils/auth/keycloak.service.ts
Module not found: Error: Can't resolve 'keycloak-js' in 'C:\Development\02_Artefakte\manual-data-collection\angular\src\app\utils\auth'


Can someone help me?

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