rustpkg: Make crates, not packages, the unit of rustpkg dependencies

r? @brson Treating a package as the thing that can have other packages depend on it,
and depends on other packages, was wrong if a package has more than one
crate. Now, rustpkg knows about dependencies between crates in the same
package. This solves the problem reported in #7879 where rustpkg wrongly
discovered a circular dependency between thhe package and itself, and
recursed infinitely.

Closes #7879