Add huak-workspace

[cnpryer]

cc #791

The goal is to simplify Workspaces. Often I need the following for different reasons:

// dir (root)
// └── project (child)
// | └── pyproject.toml
// └── pyproject.toml
let dir = TempDir::new().unwrap();
let mock = create_mock_ws(dir.as_ref());
let cwd = mock.join("package");
let ws = resolve_root(cwd, PathMarker::file("pyproject.toml"));

assert!(ws.root().exists());
assert_eq!(ws.root(), dir.path());

Reasons:

1. toolchain - The toolchain management system has a concept of scopes which tie workspaces to toolchains.
2. virtual environment - Resolving virtual environments for projects often requires resolving a workspace.
3. package management - Resolving packages requires resolving workspaces.

I'd like to have a common resolver since many things will require it.