Now that we have the PackageName type, we need to
go through API and decide what the string means.
Multiple places expect a main package, and only
some expect a subpackage. Starting from the bottom,
it is impossible to call getPackagePath with
a subpackage, as the package path of that subpackage
depends on the main package. However, we want to be
lax here, so always return the main package path.