i18n support in Angular libraries

# 🐞 Bug report

### Command (mark with an `x`)

- [ ] new
- [ ] build
- [ ] serve
- [ ] test
- [ ] e2e
- [ ] generate
- [ ] add
- [ ] update
- [ ] lint
- [x] xi18n
- [ ] run
- [ ] config
- [ ] help
- [ ] version
- [ ] doc

### Is this a regression?

No. Related functionaliy only available with new i18n capabilities of angular 9.

### Description

I want to use ng xi18n command to extract messages for i18n tags which are part of a library project which is part of a monrepo setup created by the cli. According to docs at
https://angular.io/cli/xi18n this is supposed to be possible

> ng xi18n \
> \ The name of the project to build. Can be an application or a library.

## 🔬 Minimal Reproduction

1. create project: `ng new lib-i18n`
1. add localize: `ng add @angular/localize`
1. generate library: `ng g library my-lib`
1. add i18n attribute in my-lib.component.ts
```

my-lib works!

```
1. run `ng xi18n my-lib`: Result will be
`An unhandled exception occurred: Project 'my-lib' does not support the 'extract-i18n' target.`
After adding corresponding extract-i18n target in angular.json, error output as below.

I created a minimal demo project which incorporates the scenario describe above. It is located here:
https://github.com/multimike77/lib-i18n-issue

Checkout and run `npm install`. Then run `ng xi18n my-lib` and it should produce error as below.

## 🔥 Exception or Error

An unhandled exception occurred: The "path" argument must be of type string. Received undefined


[error] TypeError [ERR_INVALID_ARG_TYPE]: The "path" argument must be of type string. Received undefined

    at validateString (internal/validators.js:117:11)

    at Object.resolve (path.js:980:7)

    at Object.getCommonConfig (/Users/.../lib-i18n/node_modules/@angular-devkit/build-angular/src/angular-cli-files/models/webpack-configs/common.js:390:24)

    at /Users/.../lib-i18n/node_modules/@angular-devkit/build-angular/src/extract-i18n/index.js:91:27

    at generateWebpackConfig (/Users/.../lib-i18n/node_modules/@angular-devkit/build-angular/src/utils/webpack-browser-config.js:62:22)

    at async Object.generateBrowserWebpackConfigFromContext (/Users/.../lib-i18n/node_modules/@angular-devkit/build-angular/src/utils/webpack-browser-config.js:137:20)

    at async execute (/Users/.../lib-i18n/node_modules/@angular-devkit/build-angular/src/extract-i18n/index.js:73:24)

## 🌍 Your Environment

Angular CLI: 9.0.4

Node: 12.16.1

OS: darwin x64


Angular: 9.0.4

... animations, cli, common, compiler, compiler-cli, core, forms

... language-service, localize, platform-browser

... platform-browser-dynamic, router

Ivy Workspace: Yes

Package                            Version

------------------------------------------------------------

@angular-devkit/architect          0.900.4

@angular-devkit/build-angular      0.900.4

@angular-devkit/build-ng-packagr   0.900.4

@angular-devkit/build-optimizer    0.900.4

@angular-devkit/build-webpack      0.900.4

@angular-devkit/core               9.0.4

@angular-devkit/schematics         9.0.4

@ngtools/webpack                   9.0.4

@schematics/angular                9.0.4

@schematics/update                 0.900.4

ng-packagr                         9.0.2

rxjs                               6.5.4

typescript                         3.7.5

webpack                            4.41.2

**Anything else relevant?**

使用 npm install 和 ng xi18n my-lib 在链接的 lib-i18n-issue 项目中复现该问题。首先阅读 @angular-devkit/build-angular/src/extract-i18n/index.js 以及堆栈跟踪中引用的 common.js 路径。当 xi18n 接受库项目并提取其 i18n 消息，且不再出现 target 或 path 错误时，即表示完成。

由索引模型根据 Issue 内容生成。