Previously, test and benchmark dependencies were handled by editing the
package description to include or exclude those stanzas before running
the dependency resolver. Test and benchmark dependencies could only be
installed for source packages because no package description is available
for named packages before dependency resolution.

Now, test and benchmark stanzas are enabled or disabled through constraints
passed to the dependency resolver. This way, we can install dependencies for
the test suites of target packages without propagating '--enable-tests'
through the entire dependency tree; i.e., tests and benchmarks, when enabled,
are built only for target packages. Later, this will allow us to
automatically run test suites and, e.g., install only upon their success.