When I started writing the rustpkg tests, task failure didn't set the
exit code properly. But bblum's work from July fixed that. Hooray! I
just didn't know about it till now.
So, now rustpkg uses exit codes in a more conventional way, and some of
the tests are simpler.
The bigger issue will be to make task failure propagate the error message.
Right now, rustpkg does most of the work in separate tasks, which means if
a task fails, rustpkg can't distinguish between different types of failure
(see #3408)
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
