This merge request is to allow directories to be treated as fasl files in Clasp Common Lisp.
Clasp Common Lisp uses C dynamic libraries as fasl files. On macOS, DWARF debug info is separated from dynamic libraries into special DWARF files that sit in the filesystem next to the dynamic libraries. These DWARF files are created by passing the dynamic library to the 'dsymutil' utility. This causes problems with atomic creation of fasls. So in Clasp we want to use directories as fasl files. /foo/bar.fasl will become a directory and on macOS will contain /foo/bar.fasl/fasl.fasl and /foo/bar.fasl/fasl.fasl.DWARF. But ASDF is currently hardcoded to only allow fasls to be files. So this merge request relaxes the requirement that fasls be files in ASDF but only for clasp by adding four #-clasp feature tests.