Let rustpkg find sources in the current working directory even if it's not a workspace

[catamorphism]

If you execute rustpkg in a non-workspace directory, and none of your RUST_PATH entries contain the package ID you specified, rustpkg should try to build sources in the current directory. (It will, however, install to the first RUST_PATH entry in this case.)

[catamorphism]

(also, rustpkg will infer the package ID in this case from the current directory)

[catamorphism]

Fixing this is blocked on me being able to ssh into the linux bot again.