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/libpipeline/package.py

@@ -34,7 +34,7 @@ class Libpipeline(AutotoolsPackage):
 
     version('1.4.2', '30cec7bcd6fee723adea6a54389f3da2')
 
-    depends_on('pkg-config', type='build')
+    depends_on('pkgconfig', type='build')
     # TODO: Add a 'test' deptype
     # See https://github.com/spack/spack/issues/1279
     # depends_on('check', type='test')