Plugin throws `Cannot read property 'replace' of undefined` when using `extends` on tsconfig

[theGlenn]

Hi,

Serverless commands throws Cannot read property 'replace' of undefined when using the extend property with local file eg. "extends": "../../tsconfig.settings.json",

Can be reproduced with the following file.

{
  "extends": "../../tsconfig.settings.json",
  "compilerOptions": {
    "module": "commonjs",
    "target": "es6",
    "lib": [ "es6", "dom" ],
    "moduleResolution": "node",
    "rootDir": "./src",
    "outDir": ".build",
    "sourceMap": true,
    "allowJs": true,
    "noImplicitAny": true,
    "noUnusedLocals": true,
    "noImplicitThis": true,
    "strictNullChecks": true,
    "noImplicitReturns": true,
    "preserveConstEnums": true,
    "suppressImplicitAnyIndexErrors": true,
    "forceConsistentCasingInFileNames": true
  },
  "exclude": [
    "node_modules",
    "build",
  ],
  "types": [ "typePatches" ]
}