🍒[6.0][SymbolGraphGen] Correctly handle exported imports in swift-symbolgraph-extract

[daniel-grumberg]

This sets up the necessary stuff to include symbols from exported imported modules in symbol graphs generated using the swift-symbolgraph-extract tool

Changes include:

Add option to provide an allow list of reexported modules for swift-symbolgraph-extract Recursively collect exported imports to allow fetching all visible Decls for symbol graph generation Enable collection of exported imports that originate from non SourceFile file units for the purpose of ModuleDecl::getDisplayDecl Resolves https://github.com/apple/swift/issues/59920 rdar://89687175

[daniel-grumberg]

@swift-ci please test

[daniel-grumberg]

Explanation: Makes swift-symbolgraph-extract respect reexported modules and track nested reexports when generating symbol graphs during a regular build Radar (and possibly SR Issue): rdar://89687175 https://github.com/apple/swift/issues/59920 Scope: Symbol Graph Generation Risk: Low does not affect regular build. Testing: Unit tests were added and ad-hoc testing on real projects. Reviewed By: @QuietMisdreavus