brew trigger: handle different packages in a single task

Once RT is merged into the main kernel, the kernel-rt packages will be included in the "main" per-arch tasks similarly to how kernel-debug is included today. Thus, our trigger mechanism using the package name and configs will be utterly useless here.

We have to

• add a new "main task package name" config option, so we can correctly trigger separate RT pipelines which need different options
• make the trigger use the option
• be able to trigger multiple pipelines per package (e.g. both "kernel" and "kernel-rt") when applicable. this also means we need to make sure the configs are unique enough that we don't trigger pipelines we should not!