Rework Spack config: keep user & site config in memory.

- User and site config are now kept separately in memory.
- Merging is done on demand when client code requests the configuration.
- Allows user/site config to be updated independently of each other by commands.
- simplifies config logic (no more tracking merged files)

.gitignore

@@ -8,3 +8,4 @@
 /etc/spackconfig
 /share/spack/dotkit
 /share/spack/modules
+/TAGS