I thought we had a bug for this, huh.

We need a way to annotate ebuilds to disable warnings where there's either a policy exemption for something (think e.g. toolchain ebuilds which often do something gnarly), or the warning is a false positive (see #359 for TarballAvailable, #214 for MissingUnpackerDep).

In previous discussions, we've agreed it'd be best to have it be ebuild-wide (so any annotations would affect the whole ebuild) instead of trying to do it per-line (given it's hard to then restrict for all checks, it'd be ugly if you had to place it on a specific line above or so, and it's not worth the complexity).