Add a ProviderIndex cache.

- Spack will check if the index needs updating, and will only parse
  all package files if it does.

- Spack tries to parse as few package files as necessary.

.gitignore

@@ -1,5 +1,7 @@
 /var/spack/stage
 /var/spack/cache
+/var/spack/repos/*/index.yaml
+/var/spack/repos/*/lock
 *.pyc
 /opt
 *~