[Serialization] Load indirect package dependencies from the current package
When loading a swiftmodule A, read its package information to tell if
the current client should load A's dependencies imports by a package
import. Only clients belonging to the same package as A should load
those dependencies, clients outside of the package likely don't have
access to those dependencies.
This is specific to swiftmodules as swiftinterfaces never display a
package-only import. Clients are unaware of package dependencies when
building against a swiftinterface.
rdar://106164813