... It didn't really break the cycle, because there were still indirect
inclusions via AudacityHeaders.h.
This reverts commit 4f78874978975b42a5ca0ffb7175d586875da629.
FileDialog goes into audacity/src/widgets and the mod-* directories go into
audacity/modules.
This leaves nothing in lib-src that isn't a 3rd-party libs or supporting
files.