An interactive experience for opening workspaces could be nice to have

Allow opening workspace for directory that already exists. E.g.

$ bst workspace open core-deps/WebKit.bst ~/Projects/WebKit
[--:--:--][][] START   Loading pipeline
[00:00:00][][] SUCCESS Loading pipeline
[--:--:--][][] START   Resolving pipeline
[00:00:00][][] SUCCESS Resolving pipeline
[--:--:--][][] START   Resolving cached state
[00:00:08][][] SUCCESS Resolving cached state
[00:00:00][][] SUCCESS Fetched 0 elements

Checkout directory is not empty: /home/mcatanzaro/Projects/WebKit

This shouldn't fail, it should just use my existing WebKit checkout.