New, cleaner package repository structure.

Package repositories now look like this:

    top-level-dir/
        repo.yaml
        packages/
            libelf/
                package.py
            mpich/
                package.py
            ...

This leaves room at the top level for additional metadata, source,
per-repo configs, indexes, etc., and it makes it easy to see that
something is a spack repo (just look for repo.yaml and packages).

var/spack/repos/builtin/packages/pkg-config/package.py

@@ -0,0 +1,17 @@
+from spack import *
+
+class PkgConfig(Package):
+    """pkg-config is a helper tool used when compiling applications and libraries"""
+    homepage = "http://www.freedesktop.org/wiki/Software/pkg-config/"
+    url      = "http://pkgconfig.freedesktop.org/releases/pkg-config-0.28.tar.gz"
+
+    version('0.28', 'aa3c86e67551adc3ac865160e34a2a0d')
+
+    parallel = False
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" %prefix, "--enable-shared")
+
+        make()
+        make("install")
+