Convert lazy singleton functions to Singleton object

- simplify the singleton pattern across the codebase
- reduce lines of code needed for crufty initialization
- reduce functions that need to mess with a global

- Singletons whose semantics changed:
  - spack.store.store() -> spack.store
  - spack.repo.path() -> spack.repo.path
  - spack.config.config() -> spack.config.config
  - spack.caches.fetch_cache() -> spack.caches.fetch_cache
  - spack.caches.misc_cache() -> spack.caches.misc_cache

var/spack/repos/builtin/packages/openspeedshop/package.py

@@ -197,7 +197,7 @@ def set_CrayLoginNode_cmakeOptions(self, spec, cmakeOptions):
         # spec['cbtf'].prefix is the login node value for this build, as
         # we only get here when building the login node components and
         # that is all that is known to spack.
-        store = spack.store.store()
+        store = spack.store
         be_ck = store.db.query_one('cbtf-krell arch=cray-CNL-haswell')
 
         # Equivalent to install-tool cmake arg: