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/icu/package.py

@@ -0,0 +1,25 @@
+from spack import *
+
+class Icu(Package):
+    """The International Components for Unicode (ICU) package is a
+       mature, widely used set of C/C++ libraries providing Unicode and
+       Globalization support for software applications. ICU is widely
+       portable and gives applications the same results on all
+       platforms."""
+    # FIXME: add a proper url for your package's homepage here.
+    homepage = "http://www.example.com"
+    url      = "http://download.icu-project.org/files/icu4c/54.1/icu4c-54_1-src.tgz"
+
+    version('54.1', 'e844caed8f2ca24c088505b0d6271bc0')
+
+
+    def url_for_version(self, version):
+        return "http://download.icu-project.org/files/icu4c/%s/icu4c-%s-src.tgz" % (
+            version, str(version).replace('.', '_'))
+
+
+    def install(self, spec, prefix):
+        with working_dir("source"):
+            configure("--prefix=%s" % prefix)
+            make()
+            make("install")