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.

[yselkowitz]

This would also help clarify some cases where the "unwanted" designation is used for build dependencies which are ultimately necessary but the team does not want to ship (even in CRB).