[SymbolGraphGen] swift-symbolgraph-extract support for emitting symbols from `@_export import`s

[franklinsch]

Previous ID	SR-15921
Radar	rdar://problem/89687175
Original Reporter	@franklinsch
Type	Improvement

Additional Detail from JIRA

Votes	0
Component/s	Swift-DocC
Labels	Improvement
Assignee	None
Priority	Medium

md5: 3bcd4958d4ed4ce5d66606c682cf8647

Issue Description:

In swiftlang/swift-docc#190, we started exporting symbols that came from @_exported import modules when emitting symbol graph files during a Swift build. This ticket tracks adding the same support for the swift-symbolgraph-extract, which didn't come for free with swiftlang/swift-docc#190.

[stevapple]

Is this going to be resolved? It’s causing some problems building the documentation with https://github.com/apple/swift-docc-plugin, which calls swift-symbol-extract through SwiftPM plugin host.