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/ruby/package.py

@@ -0,0 +1,41 @@
+from spack import *
+import spack
+import os
+
+class Ruby(Package):
+    """A dynamic, open source programming language with a focus on 
+    simplicity and productivity."""
+
+    homepage = "https://www.ruby-lang.org/"
+    url      = "http://cache.ruby-lang.org/pub/ruby/2.2/ruby-2.2.0.tar.gz"
+
+    extendable = True
+
+    version('2.2.0', 'cd03b28fd0b555970f5c4fd481700852')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+
+        make()
+        make("install")
+
+    def setup_dependent_environment(self, module, spec, ext_spec):
+        """Called before ruby modules' install() methods.  Sets GEM_HOME
+        and GEM_PATH to values appropriate for the package being built.
+
+        In most cases, extensions will only need to have one line::
+
+            gem('install', '<gem-name>.gem')
+        """
+        # Ruby extension builds have global ruby and gem functions
+        module.ruby = Executable(join_path(spec.prefix.bin, 'ruby'))
+        module.gem = Executable(join_path(spec.prefix.bin, 'gem'))
+
+        # Set GEM_PATH to include dependent gem directories
+        ruby_paths = []
+        for d in ext_spec.traverse():
+            if d.package.extends(self.spec):
+                ruby_paths.append(d.prefix)
+        os.environ['GEM_PATH'] = ':'.join(ruby_paths)
+        # The actual installation path for this gem
+        os.environ['GEM_HOME'] = ext_spec.prefix