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/folly/package.py

@@ -50,7 +50,7 @@ class Folly(AutotoolsPackage):
     depends_on('autoconf', type='build')
     depends_on('automake', type='build')
     depends_on('libtool', type='build')
-    depends_on('pkg-config', type='build')
+    depends_on('pkgconfig', type='build')
 
     # TODO: folly requires gcc 4.9+ and a version of boost compiled with
     # TODO: C++14 support (but there's no neat way to check that these