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/yasm/package.py

@@ -0,0 +1,16 @@
+from spack import *
+
+class Yasm(Package):
+    """Yasm is a complete rewrite of the NASM-2.11.06 assembler. It
+       supports the x86 and AMD64 instruction sets, accepts NASM and
+       GAS assembler syntaxes and outputs binary, ELF32 and ELF64
+       object formats."""
+    homepage = "http://yasm.tortall.net"
+    url      = "http://www.tortall.net/projects/yasm/releases/yasm-1.3.0.tar.gz"
+
+    version('1.3.0', 'fc9e586751ff789b34b1f21d572d96af')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")