Add support for Git package dependencies.

[kirevdokimov]

https://forum.unity.com/threads/git-support-on-package-manager.573673/
It is very useful to add and update dependencies via Package manager instead of import plugins as sources.

e.g

File : <ProjectName>/Assets/Packages/manifest.json

{
   "dependencies": {
      "com.google.vr": "git@github.com:googlevr/gvr-unity-sdk.git",
      ...
   }
}

I am going to resolve the issue via pull request soon.