TODO item: Get repo cache situated in the right location

[cquinn]

Corral uses two special locations for storing things:

• repo_cache: Central location (per user or workspace) for caching repositories (without indexes or workspaces)
• corral_base: Per-project location for holding the checked-out source of each dependency of the project.

Currently, these are both being put into . as:

• ./_repos
• ./_corral

I think these are good names, but we should add a flag / env var to control the repo cache location, defaulting to $HOME if possible. And, we should make sure we situate the corral_base next to the bundle.json in all cases. (ancestor of CWD or a dir passed on the CLI)