Fixed bugs for scoped packages

Thanks to @phndiaye.

Bug fixes

• The workspaces field in the root package.json now list the package locations. A bug (listing the package names instead) would have surfaced when (1) the package is scoped or (2) an end-developer passes the argument --addon-location, --test-app-location, or --test-app-name.¹
• The regular expression for computing the relative path has been updated. A bug would have occurred when the package is scoped.¹

^{1. See #17 to learn how to keep your v2 addon up-to-date.}