This PR treats package access level as exportable, preventing
internally imported types from accidentally being declared in
package decl signatures.
Added package-specific cases to ExportabilityReason and
DisallowedOriginKind to track the validity of imported types
at use sites with package access scope. Added tests to cover
variety of use cases.
Resolves rdar://117586046&125050064&124484388&124306642
We require that all stored properties in a @frozen struct have
public or @usableFromInline types, even if the property itself
is not public. This is so that clients can correctly generate
code to manipulate the @frozen struct.
This check was only looking for bona-fide stored properties,
and missing out looking at properties that have backing storage,
namely 'lazy' properties and property wrappers.
