Support marking a package as wanted but buildroot-only

[yselkowitz]

Currently, CR treats any listed package as Required and therefore part of the runtime, and only unlisted BuildRequires that aren't also runtime deps end up in Buildroot Level 1. However, in a few cases we need certain packages which are not normally BuildRequires (e.g. javapackages-bootstrap, or rpmautospec) to be tracked as a buildroot dependency even though its usually isn't directly, so that it will end up in the next RHEL version, while not implying that it will be shipped.