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/openexr/package.py

@@ -43,7 +43,7 @@ class Openexr(Package):
     variant('debug', default=False,
             description='Builds a debug version of the libraries')
 
-    depends_on('pkg-config', type='build')
+    depends_on('pkgconfig', type='build')
     depends_on('ilmbase')
 
     def install(self, spec, prefix):