In ca71b4b902 I went about fixing the
inexhaustive matching in Scope's resolveProjectBuild and
resolveProjectRef. Looking back the change was wrong.
For resolveProjectBuild the new implementation is less wrong, but still
not great, seeing as it doesn't actually do any build resolving.
For resolveProjectRef the new implementation now blows up instead of
lies. Which means it's less leneant, more "fail-fast".
isProjectThis is unused; remnant of the pre-AutoPlugin days when build
settings where defined in Plugin.settings.