Handle `rustpkg build`, etc. when given no args properly

`rustpkg build` et al were only checking one directory up to see if it
was in a dir named "src". Ditch that entirely and instead check if the
cwd is descended from any of the workspace paths. Besides being more
intelligent about whether or not something is a workspace, this also
allows for package ids composed of multiple path components.

r? @catamorphism