`pub workspace add`

New command to add a package into the current workspace.

```
> dart pub workspace add [pkgs/blah]
```

Would:

* Complain if [pkgs/blah] is already has `resolution: workspace`
* resolve [pkgs/blah] as a glob relative to the current package
* add [pkgs/blah] to the workspace of the current package.
* change all the resolved packages to have resolution: workspace
* 