Fix pkg-config dependencies (#9738)

Replace dependencies on specific pkgconfig implementation (e.g. pkgconf)
with dependency on pkgconfig virtual package

var/spack/repos/builtin/packages/axel/package.py

@@ -18,6 +18,6 @@ class Axel(AutotoolsPackage):
     depends_on('autoconf', type='build')
     depends_on('automake', type='build')
     depends_on('libtool', type='build')
-    depends_on('pkgconf', type='build')
+    depends_on('pkgconfig', type='build')
     depends_on('gettext')
     depends_on('openssl')