Disallow nesting of packages with same packge_id

Disallow nesting of packages with same packge_id

If a package contains a subdirectory that contains a copy of
itself (strange, but encountered in the wild in the context of an OS
package build tool), cargo was failing with a non-descriptive "An
unknown error occurred" (or "no package found in source" with
--verbose).

Since nested packages use the root path, such a package will have an
identical package_id to the root package. With this patch, we keep the
first occurance of a given package_id rather than overwritting --
effectively ignoring the nested package with duplicated name. A warn!
message is printed.