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/glm/package.py

@@ -0,0 +1,19 @@
+from spack import *
+
+
+class Glm(Package):
+    """
+    OpenGL Mathematics (GLM) is a header only C++ mathematics library for graphics software based on
+    the OpenGL Shading Language (GLSL) specification.
+    """
+
+    homepage = "https://github.com/g-truc/glm"
+    url = "https://github.com/g-truc/glm/archive/0.9.7.1.tar.gz"
+
+    version('0.9.7.1', '61af6639cdf652d1cdd7117190afced8')
+
+    def install(self, spec, prefix):
+        with working_dir('spack-build', create=True):
+            cmake('..', *std_cmake_args)
+            make()
+            make("install")