'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?**

Bắt đầu với phần triển khai lệnh update được liên kết trong packages/angular/cli/commands/update-impl.ts quanh dòng 123, sau đó tái hiện lỗi bằng một Yarn workspace chỉ có các dependency của Angular trong workspace-a/package.json. Xác minh cách cơ chế phân giải dependency hoạt động từ thư mục workspace; được xem là hoàn thành khi ng update tìm thấy và có thể cập nhật các dependency Angular của workspace mà không yêu cầu chúng có trong root package.json.

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