libstore-c: Organize API into separate headers

Move StorePath and Derivation declarations to their own headers in a
backwards compatible way:

- Created nix_api_store/store_path.h for StorePath operations

- Created nix_api_store/derivation.h for Derivation operations

- Main nix_api_store.h includes both headers for backwards compatibility

This reorganization improves modularity and hopefully makes the API
easier to navigate.

src/libstore-c/meson.build

@@ -35,6 +35,8 @@ include_dirs = [ include_directories('.') ]
 
 headers = files(
   'nix_api_store.h',
+  'nix_api_store/derivation.h',
+  'nix_api_store/store_path.h',
 )
 
 # TODO don't install this once tests don't use it and/or move the header into `libstore`, non-`c`