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/gflags/package.py

@@ -0,0 +1,21 @@
+import os
+from spack import *
+
+class Gflags(Package):
+    """The gflags package contains a C++ library that implements
+    commandline flags processing. It includes built-in support for
+    standard types such as string and the ability to define flags
+    in the source file in which they are used. Online documentation
+    available at: https://gflags.github.io/gflags/"""
+
+    homepage = "https://gflags.github.io/gflags"
+    url      = "https://github.com/gflags/gflags/archive/v2.1.2.tar.gz"
+
+    version('2.1.2', 'ac432de923f9de1e9780b5254884599f')
+
+    def install(self, spec, prefix):
+        cmake("-DCMAKE_INSTALL_PREFIX=" + prefix,
+              "-DBUILD_SHARED_LIBS=ON")
+        make()
+        make("test")
+        make("install")