'ng update' does not work with yarn workspaces

# 🐞 Bug report

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

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

### Is this a regression?

No

### Description

I am utilizing yarn workspaces -feature in my project. Problem is that I have to keep Angular & AngularCLI dependencies on root packages.json, otherwise 'ng update' command can't figure out the version I currently have. I'd rather keep root package.json clean and move Angular & CLI dependencies to project/package.json to keep things isolated.
Looking at the [code](https://github.com/angular/angular-cli/blob/af93227f3295e918771e018d7d0fabbef774f1d1/packages/angular/cli/commands/update-impl.ts#L123), update command resolves dependencies from root package, even when command is executed on package folder. Would it be possible to change behavior in a way that update would first check package dependencies and the fall back to root if nothing is found?

## 🔬 Minimal Reproduction

- init yarn workspace project as follows: https://yarnpkg.com/lang/en/docs/workspaces/
- init angular app with cli and move dependencies from root package.json to workspace-a/package.json (example from previous step)
- wait until new angular version is available
- cd to 'workspace-a' and run 'ng update'. Command fails to find any dependencies to update.

## 🔥 Exception or Error

N/A

## 🌍 Your Environment

Angular CLI: 8.0.3

Node: 10.15.3

OS: win32 x64

Angular: 8.0.1

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

... language-service, platform-browser, platform-browser-dynamic

... router, service-worker


Package                           Version

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

@angular-devkit/architect         0.800.3

@angular-devkit/build-angular     0.800.3

@angular-devkit/build-optimizer   0.800.3

@angular-devkit/build-webpack     0.800.3

@angular-devkit/core              8.0.3

@angular-devkit/schematics        8.0.3

@angular/cli                      8.0.3

@angular/pwa                      0.800.3

@ngtools/webpack                  8.0.3

@schematics/angular               8.0.3

@schematics/update                0.800.3

rxjs                              6.4.0

typescript                        3.4.5

webpack                           4.30.0

Root package.json:

  "dependencies": {},

  "devDependencies": {

    "npm-run-all": "^4.1.5"

  },

'workspace-a/package.json':

"dependencies": {

    "@angular/animations": "~8.0.1",

    "@angular/common": "~8.0.1",

    "@angular/compiler": "~8.0.1",

    "@angular/core": "~8.0.1",

    "@angular/forms": "~8.0.1",

    "@angular/platform-browser": "~8.0.1",

    "@angular/platform-browser-dynamic": "~8.0.1",

    "@angular/pwa": "^0.800.3",

    "@angular/router": "~8.0.1",

    "@angular/service-worker": "~8.0.1",

    "@auth0/angular-jwt": "^2.1.0",

    "@ngx-translate/core": "^11.0.1",

    ...

    "rxjs": "~6.4.0",

    "tslib": "^1.9.0",

    "zone.js": "~0.9.1"

  },

  "devDependencies": {

    "@angular-devkit/build-angular": "~0.800.0",

    "@angular/cli": "~8.0.3",

    "@angular/compiler-cli": "~8.0.1",

    "@angular/language-service": "~8.0.1",

    ...

  }

**Anything else relevant?**

先從 packages/angular/cli/commands/update-impl.ts 中約第 123 行連結的 update 命令實作開始，然後使用一個僅在 workspace-a/package.json 中包含 Angular 相依性的 Yarn workspace 重現該失敗。驗證從 workspace 目錄進行相依性解析時的行為；當 ng update 能夠找到並更新 workspace 的 Angular 相依性，且不要求它們存在於 root package.json 中時，即表示完成。

由索引模型根據 Issue 內容生成。