Junctions (linked folders) in the assets folder are only partially copied to the build output

# 🐞 Bug report

### Command

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

### Is this a regression?
Did not work with ng 8 and does not work with ng 10. Did not check with 9 or anything before 8.

### Description

When creating a junction inside the src/assets folder to another folder somewhere in the filesystem, the compiled output only contains the files that are directly beneath the linked folder. Any sub-folders in this linked folder are ignored by the compiler.

## 🔬 Minimal Reproduction
Tested on Windows 10:

1. Create an empty angular app with `ng new assetsBugApp`,
2. create a folder outside the application like "testAssets"
3. Inside "testAssets" create a textfile "works.txt"
4. Inside "testAssets" create a folder "myFolder"
5. Inside "testAssets/myFolder" create a textfile "does not work.txt"
6. Create a junction from "assetsBugApp/src/assets/test" to "testAssets" (eg `mklink /J C:\tmp\assetsBugApp\src\assets\test C:\tmp\testAssets`)
7. Run `ng build`
8. Look in the dist/assets folder. You will find the folder "test" containing "works.txt" but no "myFolder" and no "does not work.txt"

## 🌍 Your Environment

Angular CLI: 10.1.4

Node: 12.18.3

OS: win32 x64


Angular: 10.1.4

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

... elements, forms, platform-browser, platform-browser-dynamic

... router

Ivy Workspace: Yes

Package                            Version

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

@angular-devkit/architect          0.1001.4

@angular-devkit/build-angular      0.1001.4

@angular-devkit/build-ng-packagr   0.1001.4

@angular-devkit/core               10.1.1

@angular-devkit/schematics         10.1.4

@angular/cdk                       10.2.3

@angular/language-service          10.1.1

@angular/material                  10.2.3

@schematics/angular                10.1.4

@schematics/update                 0.1001.4

ng-packagr                         10.1.2

rxjs                               6.6.3

typescript                         3.9.7

**Anything else relevant?**
Might be isolated to Windows environments.

Would be great to have this resolved, copying assets from A to B is less fun than linking them... :)

Beginne damit, den Windows-10-Fall mit einer Junction unter src/assets zu reproduzieren, und vergleiche dann die Behandlung des verknüpften Ordners durch build und serve mit der Ausgabe unter dist/assets. Erledigt ist es, wenn verschachtelte Ordner und Dateien wie myFolder/does not work.txt kopiert werden und nicht nur Dateien direkt unter der Junction.

Vom Indexierungsmodell aus dem Issue-Text verfasst.