Separate path.hh from content-address.hh

It is good to separate concerns; `StorePath` (in general) has nothing to
do with `ContentAddress` anyways.

This reduces the diff from #3746.

src/libstore/store-api.hh

@@ -87,6 +87,8 @@ enum BuildMode { bmNormal, bmRepair, bmCheck };
 struct BuildResult;
 
 
+typedef std::map<StorePath, std::optional<ContentAddress>> StorePathCAMap;
+
 struct StoreConfig : public Config
 {
     using Config::Config;