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/pcre/package.py

@@ -0,0 +1,15 @@
+from spack import *
+
+class Pcre(Package):
+    """The PCRE package contains Perl Compatible Regular Expression
+       libraries. These are useful for implementing regular expression
+       pattern matching using the same syntax and semantics as Perl 5."""
+    homepage = "http://www.pcre.org"""
+    url      = "ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/pcre-8.36.tar.bz2"
+
+    version('8.36', 'b767bc9af0c20bc9c1fe403b0d41ad97')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")