Destination workspace for remote packages

[catamorphism]

rustpkg's current behavior if you have no RUST_PATH set in your environment and you execute:

mkdir Workspace/src
cd Workspace
rustpkg install github.com/foo/bar

is to create a workspace in Workspace/.rust and install bar's files into Workspace/.rust/lib (and so on).

This strikes me as a little bit silly, since if you created a workspace and cd'd into it, you probably want that to be the destination workspace for anything you are installing while you're in that directory. So this should probably be changed.

[catamorphism]

#9732 will fix this, but I don't have a specific test for it.

[catamorphism]

Needs test

[sfackler]

Rustpkg's gone