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?**

Tái hiện vấn đề trong dự án lib-i18n-issue được liên kết bằng npm install và ng xi18n my-lib. Bắt đầu bằng cách đọc @angular-devkit/build-angular/src/extract-i18n/index.js và đường dẫn common.js được tham chiếu trong stack trace. Hoàn thành khi xi18n chấp nhận một dự án thư viện và trích xuất các thông báo i18n của dự án đó mà không gặp lỗi target hoặc path.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.