This change is two fold. Firstly it enables collection of exported
imports from non source file units. Additionally this recurses through
the exported imports to ensure the transitive set is collected.
Fixes https://github.com/apple/swift/issues/59920
rdar://89687175
* only include the given symbol for qualified imports
rdar://96309088
* re-exporting one type should not allow unrelated types to sneak in
* ensure that children of re-exported types are also re-exported
When emitting a symbol graph file for a module that import modules via
`@_exported import`, emits those modules' symbols as well.
SR-15753
rdar://89547374
