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/tcl/package.py

@@ -0,0 +1,22 @@
+from spack import *
+
+class Tcl(Package):
+    """Tcl (Tool Command Language) is a very powerful but easy to
+       learn dynamic programming language, suitable for a very wide
+       range of uses, including web and desktop applications,
+       networking, administration, testing and many more. Open source
+       and business-friendly, Tcl is a mature yet evolving language
+       that is truly cross platform, easily deployed and highly
+       extensible."""
+    homepage = "http://www.tcl.tk"
+
+    version('8.6.3', 'db382feca91754b7f93da16dc4cdad1f',
+            url="http://prdownloads.sourceforge.net/tcl/tcl8.6.3-src.tar.gz")
+
+    depends_on('zlib')
+
+    def install(self, spec, prefix):
+        with working_dir('unix'):
+            configure("--prefix=%s" % prefix)
+            make()
+            make("install")