codes, dia, nest: fix pkg-config dependency (#9498)

pkgconfig is the virtual provider, pkg-config is a specific
implementation.

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

@@ -42,7 +42,7 @@ class Codes(AutotoolsPackage):
     depends_on('automake', type='build')
     depends_on('libtool', type='build')
     depends_on('m4', type='build')
-    depends_on('pkg-config', type='build')
+    depends_on('pkgconfig', type='build')
     depends_on('bison', type='build')
     depends_on('flex', type='build')
 

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

@@ -34,7 +34,7 @@ class Dia(Package):
 
     depends_on('intltool', type='build')
     depends_on('gettext', type='build')
-    depends_on('pkg-config', type='build')
+    depends_on('pkgconfig', type='build')
     depends_on('gtkplus@2.6.0:')
     depends_on('libxslt')
     depends_on('python')

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

@@ -77,7 +77,7 @@ class Nest(CMakePackage):
     depends_on('gsl', when='+gsl')
     depends_on('readline')
     depends_on('libtool')
-    depends_on('pkg-config', type='build')
+    depends_on('pkgconfig', type='build')
 
     extends('python', when='+python')