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/wx/package.py

@@ -44,7 +44,7 @@ class Wx(AutotoolsPackage):
 
     version('develop', git='https://github.com/wxWidgets/wxWidgets.git', branch='master')
 
-    depends_on('pkg-config', type='build')
+    depends_on('pkgconfig', type='build')
     depends_on('gtkplus')
 
     @when('@:3.0.2')