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/Mitos/package.py

@@ -0,0 +1,19 @@
+from spack import *
+
+class Mitos(Package):
+    """Mitos is a library and a tool for collecting sampled memory
+    performance data to view with MemAxes"""
+
+    homepage = "https://github.com/scalability-llnl/Mitos"
+    url      = "https://github.com/scalability-llnl/Mitos"
+
+    version('0.9.1', 'c6cb57f3cae54f5157affd97ef7ef79e', git='https://github.com/scalability-llnl/Mitos.git', tag='v0.9.1')
+
+    depends_on('dyninst@8.2.1:')
+    depends_on('hwloc')
+
+    def install(self, spec, prefix):
+        with working_dir('spack-build', create=True):
+            cmake('..', *std_cmake_args)
+            make()
+            make("install")