hac 0.21.0

We encountered an error during the generation of documentation:

raised LANGKIT_SUPPORT.ERRORS.PROPERTY_ERROR : BasicDecl with multiple defining names
ERROR: Command ["gnatdoc4", "-P", "hac.gpr", "--output-dir", "/home/chouteau/src/github/alire/docs.ada.dev/crate/hac/0.21.0", "--front-matter", "crate: hac", "--no-subprojects"] exited with code 1