Support different directories wit add [repo] [dir] similarly to git submodule add [repo] [dir]

[jackkoenig]

Basically it would be nice to be able to decouple the directory name from the dependency.

There is a name field in the wit-manifest.json that appears to be ignored (at least when I manually modified it).

I'm actually not sure if this is generally a good thing to take advantage of in practice. For example, how do you do transitive dependency resolution when two things depend on the same repo but use
different names for it? But the fact that repos can move and change names means that some resolution for that is probably necessary regardless.

Thoughts?

[aaronjanse]

Since at least #96, Wit has used the name field to determine the name of the folder in the workspace