Remove `Source::for_path`

Remove `Source::for_path`

This commit removes the `Source::for_path` constructor in favor of
`Workspace::load`. This prevents re-parsing manifests multiple times as Cargo
loads up as this can sometimes be an expensive operation. Instead the
`Workspace` now retains a cache of packages that can be updated as it goes
along. Finally, this should mean that we're only parsing path dependencies at
most once rather than multiple times.