Check pathnames for illegal characters (e.g. outputPath for build files)

Start by reproducing the ng build command with outputPath set to .../dist in a Windows sandbox, using the angular.json configuration described in the issue. Trace how the build handles that path and verify that an invalid pathname produces a warning or error before creating files or directories.

🐞 Bug report

Command (mark with an x)

• new
• build
• serve
• test
• e2e
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• add
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• lint
• extract-i18n
• run
• config
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Description

The Angular CLI should warn the user if a specified path is invalid for the current system.

I accidently set ".../dist" (three dots) as outputPath in angular.json. Correct is "../dist" (two dots). Running ng build created a huge mess. In the app root a folder has been created with ... as name (ending a folder name in Windows with a dot is illegal). Inside of this three dot folder all of my project files were listed (including node_modules folder). There was also a folder named ..., which again contained my whole project files. This recursively went to whatever level (I clicked throuhg the folder approximately 30 levels down, there was still a ... folder). Deleting the first ... folder lastet more than 15 minutes, then telling me the files which should be deleted aren't there anymore. Also my project files in the root directory were deleted in this process (luckily most of it in Git). That was the strangest thing, it felt like the content inside of ... mirrored my root project somehow (like in https://superuser.com/questions/528502/cant-remove-three-dots-folder-on-my-hdd). I also tried reverting my files using Git which did not work at all. I managed to delete the whole repository locally with rm -rf 'folder' using the GitBash. After cloning the project again, my IDE (Webstorm) listed the ... folder once again, which luckily could be deleted this time. The project looks fine right now, and with the correct outputPath the app builds correctly.

It would be nice if the CLI checks pathnames for illegal characters. I did not try any other illegal characters inside of the path name.

🔬 Minimal Reproduction

Caution: Try in a sandbox environment with unimported files only to prevent loss of data!

1. Change the "outputPath" in angular.json to ".../dist" (three dots)
2. Run ng build

🌍 Your Environment

Angular CLI: 11.0.1
Node: 14.15.0
OS: win32 x64

Angular: 11.0.0
... animations, common, compiler, compiler-cli, core, forms
... platform-browser, platform-browser-dynamic, router
Ivy Workspace: Yes

Package                         Version
---------------------------------------------------------
@angular-devkit/architect       0.1100.1
@angular-devkit/build-angular   0.1100.1
@angular-devkit/core            11.0.1
@angular-devkit/schematics      11.0.1
@angular/cdk                    11.0.1
@angular/cli                    11.0.1
@angular/material               11.0.1
@schematics/angular             11.0.1
@schematics/update              0.1100.1
rxjs                            6.6.3
typescript                      4.0.5

Anything else relevant?