Make storeDir a part of UnkeyedValidPathInfo

The previous commit hacked it into the output of `nix path-info --json`,
this cleans that up my making it an actual field of that data type, and
part of the canonical JSON serializers for it (and `ValidPathInfo` and
`NarInfo`).

Beyond cleaning up the JSON code, this also opens the doors to things
like:

- Binary caches that contain store objects that don't all belong in the
  same store directory

- Relocatable store objects which carefully don't mention any store
  directory by absolute path, and instead use relative paths for
  anything. (#9549)

src/libstore-tests/data/path-info/json-2/empty_impure.json

@@ -10,6 +10,7 @@
   "references": [],
   "registrationTime": null,
   "signatures": [],
+  "storeDir": "/nix/store",
   "ultimate": false,
   "version": 2
 }

src/libstore-tests/data/path-info/json-2/empty_pure.json

@@ -7,5 +7,6 @@
   },
   "narSize": 0,
   "references": [],
+  "storeDir": "/nix/store",
   "version": 2
 }

src/libstore-tests/data/path-info/json-2/impure.json

@@ -23,6 +23,7 @@
     "asdf",
     "qwer"
   ],
+  "storeDir": "/nix/store",
   "ultimate": true,
   "version": 2
 }

src/libstore-tests/data/path-info/json-2/pure.json

@@ -17,5 +17,6 @@
     "g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-bar",
     "n5wkd9frr45pa74if5gpz9j7mifg27fh-foo"
   ],
+  "storeDir": "/nix/store",
   "version": 2
 }