Introduce virtual dependency pkgconfig (#5198)

There are two providers, pkgconf and pkg-config, with the former being
the default provider.

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

@@ -62,7 +62,7 @@ class Hwloc(AutotoolsPackage):
     depends_on('cuda', when='+cuda')
     depends_on('libpciaccess', when='+pci')
     depends_on('libxml2', when='+libxml2')
-    depends_on('pkg-config', type='build')
+    depends_on('pkgconfig', type='build')
 
     def url_for_version(self, version):
         return "http://www.open-mpi.org/software/hwloc/v%s/downloads/hwloc-%s.tar.gz" % (version.up_to(2), version)