is duplicated by:

• SR-12718 SwiftPM 5.2 doesn't notice changes in header files during incremental builds

Issue Description:

Prior to SwiftPM 5.2, a change to a C header in a package would cause any source file that either directly or indirectly included the header to be recompiled.

This is no longer the case in SwiftPM 5.2

1. Create a package and include a C header and a C source file in a target
2. Make the C source file #include the C header
3. Build
4. Change the header
5. Build (without cleaning)

Notice that the source file isn't recompiled.