Add support for Github Package Registry

Hello,

Long time lurker first time poster. I really love the direction Unity is going with the package manager but with the advent of some of the developments with Github I would like to see the package manager not only support git urls but it should now support the Github Package Registry.

Here is an example unity library I wrote and here is what the package.json need to look like to beable to publish the resulting "package" to github - https://github.com/red-owl-games/UIEX/blob/a1a2696555a585feda73640ac3847668a051fe11/package.json

You can view the package here - https://github.com/red-owl-games/UIEX/packages/32700
and here is the github documentation on configuring NPM to download that package "unauth'd" - https://help.github.com/en/github/m...use-with-github-packages#installing-a-package

That being said - i've tried and it seems to come down to the fact that unity is forcing a "domain reverse lookup" style on the naming of packages.

When i configure my project manifest.json file like so

Code (JavaScript):

1. {
2.   "scopedRegistries": [
3.     {
4.       "name": "RedOwl",
5.       "url": "https://npm.pkg.github.com/red-owl-games",
6.       "scopes": [
7.         "com.redowlgames"
8.       ]
9.     }
10.   ],
11.   "dependencies": {
12.     "com.redowlgames.uiex": "1.0.0"
13.   }
14. }

I get the error

Code (CSharp):

1. An error occurred while resolving packages:
2.   Project has invalid dependencies:
3.     com.redowlgames.uiex: Request [GET https://npm.pkg.github.com/red-owl-games/com.redowlgames.uiex] failed with status code [401]

Since you guys seem to be trying to closely (but no 100% adhere to the NPM spec) it seems like you could easily support this by allowing the following

Code (JavaScript):

1. {
2.   "scopedRegistries": [
3.     {
4.       "name": "RedOwl",
5.       "url": "https://npm.pkg.github.com/red-owl-games",
6.       "scopes": [
7.         "@red-owl-games"
8.       ]
9.     }
10.   ],
11.   "dependencies": {
12.     "@red-owl-games/uiex": "1.0.0"
13.   }
14. }

and thus building the request as https://npm.pkg.github.com/red-owl-games/uiex

but doing the above results in the seemingly forced error on Unity's side of

Code (CSharp):

1.  
2. An error occurred while resolving packages:
3.   Project has invalid dependencies:
4.     @red-owl-games/uiex: Package name '@red-owl-games/uiex' is invalid.
5.  

If Unity package manager would support Github Package Registry - it would be a huge boon to opensource package managers (and likely Unity itself could use it because of the number of new github based projects unity is pushing)

Cheers!