Handle rustpkg build, etc. when given no args properly

[lilyball]

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

[lilyball]

Tests failed because rustpkg did not consider the temporary dir to be a workspace. The old logic for "no args" used to just check if the parent dir was src, but I changed that logic to specifically check if we're inside a workspace.

If I edit the test to add a .rust directory inside of the temporary directory, this causes the test to pass. I assume that this test modification is appropriate in light of the new behavior.

[catamorphism]

Yes, the test modification is okay. Thanks!