Replace pkg-config dependencies with virtual (#8783)

Several packages specified pkg-config as a dependency, which is
a specific implementation of the pkgconfig virtual. Most packages
do not request a specific implementation. The current concretizer
may choose a different implentation of pkgconfig for the nonspecific
dependents, which conflicts with the request for pkg-config. This
replaces all requests for a specific implementation of pkgconfig
with the virtual package as a temporary solution to the issue.

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

@@ -38,7 +38,7 @@ class Fluxbox(AutotoolsPackage):
 
     version('1.3.7', 'd99d7710f9daf793e0246dae5304b595')
 
-    depends_on('pkg-config', type='build')
+    depends_on('pkgconfig', type='build')
     depends_on('freetype')
     depends_on('libxrender')
     depends_on('libxext')