i18n support in Angular libraries

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。

🐞 Bug report

Command (mark with an x)

• new
• build
• serve
• test
• e2e
• generate
• add
• update
• lint
• 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 <project>
<project> The name of the project to build. Can be an application or a library.

🔬 Minimal Reproduction

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

   <p i18n>
     my-lib works!
   </p>
   

5. 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?